postgis/doc/postgis.xml

<?xml version="1.0" encoding="UTF-8"?>
<book>
  <title>PostGIS Manual</title>

  <bookinfo>
    <editor>
      <firstname>Paul</firstname>

      <surname>Ramsey</surname>

      <affiliation>
        <orgname><ulink url="http://www.refractions.net">Refractions Research
        Inc</ulink></orgname>

        <address>
     <street>400 - 1207 Douglas Street</street>
     <city>Victoria</city>
     <state>British Columbia</state>
     <country>Canada</country>
     <email>pramsey@refractions.net</email>
    </address>
      </affiliation>
    </editor>

    <abstract>
      <para>PostGIS is an extension to the PostgreSQL object-relational
      database system which allows GIS (Geographic Information Systems)
      objects to be stored in the database. PostGIS includes support for
      GiST-based R-Tree spatial indexes, and functions for basic analysis of
      GIS objects.</para>
    </abstract>
  </bookinfo>

  <chapter>
    <title>Introduction</title>

    <para>PostGIS is developed by Refractions Research Inc, as a spatial
    database technology research project. Refractions is a GIS and database
    consulting company in Victoria, British Columbia, Canada, specializing in
    data integration and custom software development. We plan on supporting
    and developing PostGIS to support a range of important GIS functionality,
    including full OpenGIS support, advanced topological constructs
    (coverages, surfaces, networks), desktop user interface tools for viewing
    and editing GIS data, and web-based access tools.</para>

    <sect1>
      <title>Credits</title>

      <variablelist>
        <varlistentry>
          <term>Dave Blasby &lt;dblasby@refractions.net&gt;</term>

          <listitem>
            <para>The principal developer of PostGIS. Dave maintains the
            server side objects and index support, the server side analytical
            functions, and the Mapserver connectivity.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Chris Hodgson &lt;chodgson@refractions.net&gt;</term>

          <listitem>
            <para>Maintains new functions and the 7.2 index bindings.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Paul Ramsey &lt;pramsey@refractions.net&gt;</term>

          <listitem>
            <para>Maintains the JDBC objects and keeps track of the
            documentation and packaging.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Jeff Lounsbury &lt;jeffloun@refractions.net&gt;</term>

          <listitem>
            <para>Maintains the Shape loader/dumper.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect1>

    <sect1>
      <title>More Information</title>

      <itemizedlist>
        <listitem>
          <para>The latest software, documentation and news items are
          available at the PostGIS web site, <ulink
          url="http://postgis.refractions.net">http://postgis.refractions.net</ulink>.</para>
        </listitem>

        <listitem>
          <para>More information about the GEOS geometry operations library is
          available at<ulink url="http://www.remotesensing.org/proj">
          http://geos.refractions.net</ulink>.</para>
        </listitem>

        <listitem>
          <para>More information about the Proj4 reprojection library is
          available at <ulink
          url="http://www.remotesensing.org/proj">http://www.remotesensing.org/proj</ulink>.</para>
        </listitem>

        <listitem>
          <para>More information about the PostgreSQL database server is
          available at the PostgreSQL main site <ulink
          url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>
        </listitem>

        <listitem>
          <para>More information about GiST indexing is available at the
          PostgreSQL GiST development site, <ulink
          url="http://www.sai.msu.su/~megera/postgres/gist">http://www.sai.msu.su/~megera/postgres/gist</ulink>.</para>
        </listitem>

        <listitem>
          <para>More information about Mapserver internet map server is
          available at <ulink
          url="http://mapserver.gis.umn.edu/">http://mapserver.gis.umn.edu</ulink>.</para>
        </listitem>

        <listitem>
          <para>The "<ulink
          url="http://www.opengis.org/techno/specs/99-049.pdf">Simple Features
          for Specification for SQL</ulink>" is available at the OpenGIS
          Consortium web site: <ulink
          url="http://www.opengis.org">http://www.opengis.org</ulink>.</para>
        </listitem>
      </itemizedlist>
    </sect1>
  </chapter>

  <chapter>
    <title>Installation</title>

    <sect1>
      <title>Requirements</title>

      <para>PostGIS has the following requirements for building and
      usage:</para>

      <itemizedlist>
        <listitem>
          <para>A complete configured and built PostgreSQL source code tree.
          PostGIS uses definitions from the PostgreSQL configure/build process
          to conform to the particular platform you are building on.
          PostgreSQL is available from <ulink
          url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>
        </listitem>

        <listitem>
          <para>GNU C compiler (<filename>gcc</filename>). Some other ANSI C
          compilers can be used to compile PostGIS, but we find far fewer
          problems when compiling with <filename>gcc</filename>.</para>
        </listitem>

        <listitem>
          <para>GNU Make (<filename>gmake</filename> or
          <filename>make</filename>). For many systems, GNU
          <filename>make</filename> is the default version of make. Check the
          version by invoking <filename>make -v</filename>. Other versions of
          <filename>make</filename> may not process the PostGIS
          <filename>Makefile</filename> properly.</para>
        </listitem>

        <listitem>
          <para>(Recommended) Proj4 reprojection library. The Proj4 library is
          used to provide coordinate reprojection support within PostGIS.
          Proj4 is available for download from <ulink
          url="http://www.remotesensing.org/proj">http://www.remotesensing.org/proj</ulink>.</para>
        </listitem>

        <listitem>
          <para>(Recommended) GEOS geometry library. The GEOS library is used
          to provide geometry tests (Touches(), Contains(), Intersects()) and
          operations (Buffer(), GeomUnion(), Difference()) within PostGIS.
          GEOS is available for download from <ulink
          url="http://www.remotesensing.org/proj">http://geos.refractions.net</ulink>.</para>
        </listitem>
      </itemizedlist>
    </sect1>

    <sect1 id="PGInstall">
      <title>PostGIS</title>

      <para>The PostGIS module is a extension to the PostgreSQL backend
      server. As such, PostGIS @@LAST_RELEASE_VERSION@@
      <emphasis>requires</emphasis> a full copy of the PostgreSQL source tree
      in order to compile. The PostgreSQL source code is available at <ulink
      url="http://www.postgresql.org">http://www.postgresql.org</ulink>.</para>

      <para>PostGIS @@LAST_RELEASE_VERSION@@ can be built against PostgreSQL
      versions 7.1.0 to 7.4.x. Earlier versions of PostgreSQL are
      <emphasis>not</emphasis> supported.</para>

      <orderedlist>
        <listitem>
          <para>Before you can compile the PostGIS server modules, you must
          compile and install the PostgreSQL package.</para>

          <note>
            <para>If you plan to use GEOS functionality you might need to
            explicitly link PostgreSQL against the standard C++
            library:</para>

            <programlisting>LDFLAGS=-lstdc++ ./configure [YOUR OPTIONS HERE]</programlisting>

            <para>This is a workaround for bogus C++ exceptions interaction
            with older development tools. If you experience weird problems
            (backend unexpectedly closed or similar things) try this trick.
            This will require recompiling your PostgreSQL from scratch, of
            course.</para>
          </note>
        </listitem>

        <listitem>
          <para>Retrieve the PostGIS source archive from <ulink
          url="http://postgis.refractions.net/postgis-@@LAST_RELEASE_VERSION@@.tar.gz">http://postgis.refractions.net/postgis-@@LAST_RELEASE_VERSION@@.tar.gz</ulink>.
          Uncompress and untar the archive in the "contrib" directory of the
          PostgreSQL source tree.</para>

          <programlisting># cd [postgresql source tree]/contrib 
# gzip -d -c postgis-@@LAST_RELEASE_VERSION@@.tar.gz | tar xvf -</programlisting>
        </listitem>

        <listitem>
          <para>Once your PostgreSQL installation is up-to-date, enter the
          "postgis" directory, and edit the
          <filename>Makefile</filename>.</para>

          <itemizedlist>
            <listitem>
              <para>If want support for coordinate reprojection you must have
              the Proj4 library installed, set the <varname>USE_PROJ</varname>
              variable to <emphasis>1</emphasis>, and adjust the
              <varname>PROJ_DIR</varname> variable to point to your Proj4
              installation directory.</para>
            </listitem>

            <listitem>
              <para>If want to use GEOS functionality you must have the GEOS
              library installed, set the <varname>USE_GEOS</varname> variable
              to <emphasis>1</emphasis>, and adjust the
              <varname>GEOS_DIR</varname> variable to point to your GEOS
              installation directory.</para>
            </listitem>
          </itemizedlist>
        </listitem>

        <listitem>
          <para>Run the compile and install commands.</para>

          <programlisting># make 
# make install</programlisting>

          <para>All files are installed relative to the PostgreSQL install
          directory, <filename>[prefix]</filename>.</para>

          <itemizedlist>
            <listitem>
              <para>Libraries are installed
              <filename>[prefix]/lib/contrib</filename>.</para>
            </listitem>

            <listitem>
              <para>Important support files such as
              <filename>postgis.sql</filename> are installed in
              <filename>[prefix]/share/contrib</filename>.</para>
            </listitem>

            <listitem>
              <para>Loader and dumber binaries are installed in
              <filename>[prefix]/bin</filename>.</para>
            </listitem>
          </itemizedlist>
        </listitem>

        <listitem>
          <para>PostGIS requires the PL/pgSQL procedural language extension.
          Before loading the <filename>postgis.sql</filename> file, you must
          first enable PL/pgSQL. You should use the
          <filename>createlang</filename> command. The PostgreSQL 7.1
          Programmer's Guide has the details if you want to this manually for
          some reason.</para>

          <programlisting># createlang plpgsql [yourdatabase]</programlisting>
        </listitem>

        <listitem>
          <para>Now load the PostGIS object and function definitions into your
          database by loading the <filename>postgis.sql</filename> definitions
          file.</para>

          <programlisting># psql -d [yourdatabase] -f postgis.sql</programlisting>

          <para>The PostGIS server extensions are now loaded and ready to
          use.</para>
        </listitem>

        <listitem>
          <para>For a complete set of EPSG coordinate system definition
          identifiers, you can also load the
          <filename>spatial_ref_sys.sql</filename> definitions file and
          populate the <varname>SPATIAL_REF_SYS</varname> table.</para>

          <programlisting># psql -d [yourdatabase] -f spatial_ref_sys.sql</programlisting>
        </listitem>
      </orderedlist>

      <sect2>
        <title>Upgrading</title>

        <para>Upgrading PostGIS can be tricky, because the underlying C
        libraries which support the object types and geometries may have
        changed between versions. To avoid problems when upgrading, you will
        have to dump all the tables in your database, destroy the database,
        create a new one, execute the new <filename>postgis.sql</filename>
        file, then upload your database dump:</para>

        <programlisting># pg_dump -t spatialtable -f dumpfile.sql yourdatabase
# dropdb yourdatabase
# createdb yourdatabase
# createlang plpgsql yourdatabse
# psql -f postgis.sql -d yourdatabase
# psql -f dumpfile.sql -d yourdatabase
# vacuumdb -z yourdatabase</programlisting>
      </sect2>

      <sect2>
        <title>Common Problems</title>

        <para>There are several things to check when your installation or
        upgrade doesn't go as you expected.</para>

        <orderedlist>
          <listitem>
            <para>It is easiest if you untar the PostGIS distribution into the
            contrib directory under the PostgreSQL source tree. However, if
            this is not possible for some reason, you can set the
            <varname>PGSQL_SRC</varname> environment variable to the path to
            the PostgreSQL source directory. This will allow you to compile
            PostGIS, but the <command>make install</command> may not work, so
            be prepared to copy the PostGIS library and executable files to
            the appropriate locations yourself.</para>
          </listitem>

          <listitem>
            <para>Check that you you have installed PostgreSQL 7.1 or newer,
            and that you are compiling against the same version of the
            PostgreSQL source as the version of PostgreSQL that is running.
            Mix-ups can occur when your (Linux) distrubution has already
            installed PostgreSQL, or you have otherwise installed PostgreSQL
            before and forgotten about it. PostGIS will only work with
            PostgreSQL 7.1 or newer, and strange, unexpected error messages
            will result if you use an older version. To check the version of
            PostgreSQL which is running, connect to the database using psql
            and run this query:</para>

            <programlisting>SELECT version();</programlisting>

            <para>If you are running an RPM based distribution, you can check
            for the existence of pre-installed packages using the
            <command>rpm</command> command as follows: <command>rpm -qa | grep
            postgresql</command></para>
          </listitem>
        </orderedlist>

        <para>Also check that you have made any necessary changes to the top
        of the Makefile. This includes:</para>

        <orderedlist>
          <listitem>
            <para>If you want to be able to do coordinate reprojections, you
            must install the Proj4 library on your system, set the
            <varname>USE_PROJ</varname> variable to 1 and the
            <varname>PROJ_DIR</varname> to your installation prefix in the
            Makefile.</para>
          </listitem>

          <listitem>
            <para>If you want to be able to use GEOS functions you must
            install the GEOS library on your system, and set the
            <varname>USE_GEOS</varname> to 1 and the
            <varname>GEOS_DIR</varname> to your installation prefix in the
            Makefile.</para>
          </listitem>
        </orderedlist>
      </sect2>
    </sect1>

    <sect1>
      <title>JDBC</title>

      <para>The JDBC extensions provide Java objects corresponding to the
      internal PostGIS types. These objects can be used to write Java clients
      which query the PostGIS database and draw or do calculations on the GIS
      data in PostGIS.</para>

      <orderedlist>
        <listitem>
          <para>Enter the <filename>jdbc</filename> sub-directory of the
          PostGIS distribution.</para>
        </listitem>

        <listitem>
          <para>Edit the <filename>Makefile</filename> to provide the correct
          paths of your java compiler (<varname>JAVAC</varname>) and
          interpreter (<varname>JAVA</varname>).</para>
        </listitem>

        <listitem>
          <para>Run the <filename>make</filename> command. Copy the
          <filename>postgis.jar</filename> file to wherever you keep your java
          libraries.</para>
        </listitem>
      </orderedlist>
    </sect1>

    <sect1>
      <title>Loader/Dumper</title>

      <para>The data loader and dumper are built and installed automatically
      as part of the PostGIS build. To build and install them manually:</para>

      <programlisting># cd postgis-@@LAST_RELEASE_VERSION@@/loader 
# make
# make install</programlisting>

      <para>The loader is called <filename>shp2pgsql</filename> and converts
      ESRI Shape files into SQL suitable for loading in PostGIS/PostgreSQL.
      The dumper is called <filename>pgsql2shp</filename> and converts PostGIS
      tables into ESRI Shape files.</para>
    </sect1>
  </chapter>

  <chapter>
    <title>Frequently Asked Questions</title>

    <qandaset>
      <qandaentry>
        <question>
          <para>What kind of geometric objects can I store?</para>
        </question>

        <answer>
          <para>You can store point, line, polygon, multipoint, multiline,
          multipolygon, and geometrycollections. These are specified in the
          Open GIS Well Known Text Format (with 3d extentions).</para>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>How do I insert a GIS object into the database?</para>
        </question>

        <answer>
          <para>First, you need to create a table with a column of type
          "geometry" to hold your GIS data. Connect to your database with
          <filename>psql</filename> and try the following SQL:</para>

          <programlisting>CREATE TABLE gtest ( ID int4, NAME varchar(20) );
SELECT AddGeometryColumn('dbname','gtest','geom',-1,'LINESTRING',2);</programlisting>

          <para>If the geometry column addition fails, you probably have not
          loaded the PostGIS functions and objects into this database. See the
          <link linkend="PGInstall">installation instructions</link>.</para>

          <para>Then, you can insert a geometry into the table using a SQL
          insert statement. The GIS object itself is formatted using the
          OpenGIS Consortium "well-known text" format:</para>

          <programlisting>INSERT INTO gtest (ID, NAME, GEOM) VALUES (1, 'First Geometry', GeometryFromText('LINESTRING(2 3,4 5,6 5,7 8)', -1));</programlisting>

          <para>For more information about other GIS objects, see the <link
          linkend="RefObject">object reference</link>.</para>

          <para>To view your GIS data in the table:</para>

          <programlisting>SELECT id, name, AsText(geom) AS geom FROM gtest;</programlisting>

          <para>The return value should look something like this:</para>

          <programlisting> id | name           | geom
----+----------------+-----------------------------
  1 | First Geometry | LINESTRING(2 3,4 5,6 5,7 8) 
(1 row)</programlisting>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>How do I construct a spatial query?</para>
        </question>

        <answer>
          <para>The same way you construct any other database query, as an SQL
          combination of return values, functions, and boolean tests.</para>

          <para>For spatial queries, there are two issues that are important
          to keep in mind while constructing your query: is there a spatial
          index you can make use of; and, are you doing expensive calculations
          on a large number of geometries.</para>

          <para>In general, you will want to use the "intersects operator"
          (&amp;&amp;) which tests whether the bounding boxes of features
          intersect. The reason the &amp;&amp; operator is useful is because
          if a spatial index is available to speed up the test, the &amp;&amp;
          operator will make use of this. This can make queries much much
          faster.</para>

          <para>You will also make use of spatial functions, such as
          Distance(), Intersects(), Contains() and Within(), among others, to
          narrow down the results of your search. Most spatial queries include
          both an indexed test and a spatial function test. The index test
          serves to limit the number of return tuples to only tuples that
          <emphasis>might</emphasis> meet the condition of interest. The
          spatial functions are then use to test the condition exactly.</para>

          <programlisting>SELECT id, the_geom FROM thetable
WHERE
  the_geom &amp;&amp; 'POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))'
AND
  Contains(the_geom,'POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))';</programlisting>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>How do I speed up spatial queries on large tables?</para>
        </question>

        <answer>
          <para>Fast queries on large tables is the <emphasis>raison
          d'etre</emphasis> of spatial databases (along with transaction
          support) so having a good index is important.</para>

          <para>To build a spatial index on a table with a
          <varname>geometry</varname> column, use the "CREATE INDEX" function
          as follows:</para>

          <programlisting>CREATE INDEX [indexname] ON [tablename]  
  USING GIST ( [geometrycolumn] );</programlisting>

          <para>The "USING GIST" option tells the server to use a GiST
          (Generalized Search Tree) index.</para>

          <note>
            <para>For PostgreSQL version 7.1.x, you must specifically request
            a "lossy" index by appending WITH (ISLOSSY) to the index creation
            command. For PostgreSQL 7.2.x and above all GiST indexes are
            assumed to be lossy. Lossy indexes uses a proxy object (in the
            spatial case, a bounding box) for building the index.</para>
          </note>

          <para>You should also ensure that the PostgreSQL query planner has
          enough information about your index to make rational decisions about
          when to use it. To do this, you have to "gather statistics" on your
          geometry tables.</para>

          <para>For PostgreSQL 8.0.x and greater, just run the <command>VACUUM
          ANALYZE</command> command.</para>

          <para>For PostgreSQL 7.4.x and below, run the <command>SELECT
          UPDATE_GEOMETRY_STATS()</command> command.</para>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>Why aren't PostgreSQL R-Tree indexes supported?</para>
        </question>

        <answer>
          <para>Early versions of PostGIS used the PostgreSQL R-Tree indexes.
          However, PostgreSQL R-Trees have been completely discarded since
          version 0.6, and spatial indexing is provided with an
          R-Tree-over-GiST scheme.</para>

          <para>Our tests have shown search speed for native R-Tree and GiST
          to be comparable. Native PostgreSQL R-Trees have two limitations
          which make them undesirable for use with GIS features (note that
          these limitations are due to the current PostgreSQL native R-Tree
          implementation, not the R-Tree concept in general):</para>

          <itemizedlist>
            <listitem>
              <para>R-Tree indexes in PostgreSQL cannot handle features which
              are larger than 8K in size. GiST indexes can, using the "lossy"
              trick of substituting the bounding box for the feature
              itself.</para>
            </listitem>

            <listitem>
              <para>R-Tree indexes in PostgreSQL are not "null safe", so
              building an index on a geometry column which contains null
              geometries will fail.</para>
            </listitem>
          </itemizedlist>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>Why should I use the <varname>AddGeometryColumn()</varname>
          function and all the other OpenGIS stuff?</para>
        </question>

        <answer>
          <para>If you do not want to use the OpenGIS support functions, you
          do not have to. Simply create tables as in older versions, defining
          your geometry columns in the CREATE statement. All your geometries
          will have SRIDs of -1, and the OpenGIS meta-data tables will
          <emphasis>not</emphasis> be filled in properly. However, this will
          cause most applications based on PostGIS to fail, and it is
          generally suggested that you do use
          <varname>AddGeometryColumn()</varname> to create geometry
          tables.</para>

          <para>Mapserver is one application which makes use of the
          <varname>geometry_columns</varname> meta-data. Specifically,
          Mapserver can use the SRID of the geometry column to do on-the-fly
          reprojection of features into the correct map projection.</para>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>What is the best way to find all objects within a radius of
          another object?</para>
        </question>

        <answer>
          <para>To use the database most efficiently, it is best to do radius
          queries which combine the radius test with a bounding box test: the
          bounding box test uses the spatial index, giving fast access to a
          subset of data which the radius test is then applied to.</para>

          <para>The <varname>Expand()</varname> function is a handy way of
          enlarging a bounding box to allow an index search of a region of
          interest. The combination of a fast access index clause and a slower
          accurate distance test provides the best combination of speed and
          precision for this query.</para>

          <para>For example, to find all objects with 100 meters of POINT(1000
          1000) the following query would work well:</para>

          <programlisting>SELECT * 
FROM GEOTABLE 
WHERE 
  GEOCOLUMN &amp;&amp; Expand(GeometryFromText('POINT(1000 1000)',-1),100)
AND
  Distance(GeometryFromText('POINT(1000 1000)',-1),GEOCOLUMN) &lt; 100;</programlisting>
        </answer>
      </qandaentry>

      <qandaentry>
        <question>
          <para>How do I perform a coordinate reprojection as part of a
          query?</para>
        </question>

        <answer>
          <para>To perform a reprojection, both the source and destination
          coordinate systems must be defined in the SPATIAL_REF_SYS table, and
          the geometries being reprojected must already have an SRID set on
          them. Once that is done, a reprojection is as simple as referring to
          the desired destination SRID.</para>

          <programlisting>SELECT Transform(GEOM,4269) FROM GEOTABLE;</programlisting>
        </answer>
      </qandaentry>
    </qandaset>
  </chapter>

  <chapter>
    <title>Using PostGIS</title>

    <sect1 id="RefObject">
      <title>GIS Objects</title>

      <para>The GIS objects supported by PostGIS are the "Simple Features"
      defined by the OpenGIS Consortium (OGC). Note that PostGIS currently
      supports the features and the representation APIs, but not the various
      comparison and convolution operators given in the OGC "Simple Features
      for SQL" specification.</para>

      <para>Examples of the text representations of the features are as
      follows:</para>

      <itemizedlist>
        <listitem>
          <para>POINT(0 0 0)</para>
        </listitem>

        <listitem>
          <para>LINESTRING(0 0,1 1,1 2)</para>
        </listitem>

        <listitem>
          <para>POLYGON((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2 0,1 2
          0,1 1 0))</para>
        </listitem>

        <listitem>
          <para>MULTIPOINT(0 0 0,1 2 1)</para>
        </listitem>

        <listitem>
          <para>MULTILINESTRING((0 0 0,1 1 0,1 2 1),(2 3 1,3 2 1,5 4
          1))</para>
        </listitem>

        <listitem>
          <para>MULTIPOLYGON(((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2
          0,1 2 0,1 1 0)),((-1 -1 0,-1 -2 0,-2 -2 0,-2 -1 0,-1 -1 0)))</para>
        </listitem>

        <listitem>
          <para>GEOMETRYCOLLECTION(POINT(2 3 9),LINESTRING((2 3 4,3 4
          5)))</para>
        </listitem>
      </itemizedlist>

      <para>Note that in the examples above there are features with both
      2-dimensional and 3-dimensional coordinates. PostGIS supports both 2d
      and 3d coordinates -- if you describe a feature with 2D coordinates when
      you insert it, the database will return that feature to you with 2D
      coordinates when you extract it. See the sections on the <link
      linkend="force_2d">force_2d()</link> and <link
      linkend="force_3d">force_3d()</link> functions for information on
      converting features to a particular coordinate dimension
      representation.</para>

      <sect2>
        <title>Standard versus Canonical Forms</title>

        <para>The OpenGIS specification defines two standard ways of
        expressing spatial objects: the Well-Known Text (WKT) form (shown in
        the previous section) and the Well-Known Binary (WKB) form. Both WKT
        and WKB include information about the type of the object and the
        coordinates which form the object.</para>

        <para>However, the OpenGIS specification also requires that the
        internal storage format of spatial objects include a spatial
        referencing system identifier (SRID). The SRID is required when
        creating spatial objects for insertion into the database. For example,
        a valid insert statement to create and insert a spatial object would
        be:</para>

        <programlisting>INSERT INTO SPATIALTABLE ( THE_GEOM, THE_NAME ) 
VALUES ( 
    GeometryFromText('POINT(-126.4 45.32)', 312), 
    'A Place' 
  )</programlisting>

        <para>Note that the "GeometryFromText" function requires an SRID
        number.</para>

        <para>The "canonical form" of the spatial objects in PostgreSQL is a
        text representation which includes all the information necessary to
        construct the object. Unlike the OpenGIS standard forms, it includes
        the type, coordinate, and SRID information. The canonical form is the
        default form returned from a SELECT query. The example below shows the
        difference between the OGC standard and PostGIS canonical
        forms:</para>

        <programlisting>db=&gt; SELECT AsText(geom) AS OGCGeom FROM thetable;
OGCGeom
-------------------------------------------------
LINESTRING(-123.741378393049 48.9124018962261,-123.741587115639 48.9123981907507)
(1 row)

db=&gt; SELECT geom AS PostGISGeom FROM thetable;
PostGISGeom
-------------------------------------------------
SRID=123;LINESTRING(-123.741378393049 48.9124018962261,-123.741587115639 48.9123981907507)
(1 row)</programlisting>
      </sect2>
    </sect1>

    <sect1>
      <title>Using OpenGIS Standards</title>

      <para>The OpenGIS "Simple Features Specification for SQL" defines
      standard GIS object types, the functions required to manipulate them,
      and a set of meta-data tables. In order to ensure that meta-data remain
      consistent, operations such as creating and removing a spatial column
      are carried out through special procedures defined by OpenGIS.</para>

      <para>There are two OpenGIS meta-data tables: SPATIAL_REF_SYS and
      GEOMETRY_COLUMNS. The SPATIAL_REF_SYS table holds the numeric IDs and
      textual descriptions of coordinate systems used in the spatial
      database.</para>

      <sect2>
        <title>The SPATIAL_REF_SYS Table</title>

        <para>The SPATIAL_REF_SYS table definition is as follows:</para>

        <programlisting>CREATE TABLE SPATIAL_REF_SYS ( 
  SRID INTEGER NOT NULL PRIMARY KEY, 
  AUTH_NAME VARCHAR(256), 
  AUTH_SRID INTEGER, 
  SRTEXT VARCHAR(2048), 
  PROJ4TEXT VARCHAR(2048)
)</programlisting>

        <para>The SPATIAL_REF_SYS columns are as follows:</para>

        <variablelist>
          <varlistentry>
            <term>SRID</term>

            <listitem>
              <para>An integer value that uniquely identifies the Spatial
              Referencing System within the database.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>AUTH_NAME</term>

            <listitem>
              <para>The name of the standard or standards body that is being
              cited for this reference system. For example, "EPSG" would be a
              valid AUTH_NAME.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>AUTH_SRID</term>

            <listitem>
              <para>The ID of the Spatial Reference System as defined by the
              Authority cited in the AUTH_NAME. In the case of EPSG, this is
              where the EPSG projection code would go.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>SRTEXT</term>

            <listitem>
              <para>The Well-Known Text representation of the Spatial
              Reference System. An example of a WKT SRS representation
              is:</para>

              <programlisting>PROJCS["NAD83 / UTM Zone 10N", 
  GEOGCS["NAD83",
    DATUM["North_American_Datum_1983", 
      SPHEROID["GRS 1980",6378137,298.257222101]
    ], 
    PRIMEM["Greenwich",0], 
    UNIT["degree",0.0174532925199433] 
  ],
  PROJECTION["Transverse_Mercator"], 
  PARAMETER["latitude_of_origin",0],
  PARAMETER["central_meridian",-123], 
  PARAMETER["scale_factor",0.9996],
  PARAMETER["false_easting",500000], 
  PARAMETER["false_northing",0],
  UNIT["metre",1] 
]</programlisting>

              <para>For a listing of EPSG projection codes and their
              corresponding WKT representations, see <ulink
              url="http://www.opengis.org/techno/interop/EPSG2WKT.TXT">http://www.opengis.org/techno/interop/EPSG2WKT.TXT</ulink>.
              For a discussion of WKT in general, see the OpenGIS "Coordinate
              Transformation Services Implementation Specification" at <ulink
              url="http://www.opengis.org/techno/specs.htm">http://www.opengis.org/techno/specs.htm</ulink>.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>PROJ4TEXT</term>

            <listitem>
              <para>PostGIS uses the Proj4 library to provide coordinate
              transformation capabilities. The <varname>PROJ4TEXT</varname>
              column contains the Proj4 coordinate definition string for a
              particular SRID. For example:</para>

              <programlisting>+proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m</programlisting>

              <para>For more information about, see the Proj4 web site at
              <ulink
              url="http://www.remotesensing.org/proj">http://www.remotesensing.org/proj</ulink>.
              The <filename>spatial_ref_sys.sql</filename> file contains both
              <varname>SRTEXT</varname> and <varname>PROJ4TEXT</varname>
              definitions for all EPSG projections.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect2>

      <sect2>
        <title>The GEOMETRY_COLUMNS Table</title>

        <para>The <varname>GEOMETRY_COLUMNS</varname> table definition is as
        follows:</para>

        <programlisting>CREATE TABLE GEOMETRY_COLUMNS ( 
  F_TABLE_CATALOG VARCHAR(256) NOT NULL, 
  F_TABLE_SCHEMA VARCHAR(256) NOT NULL, 
  F_TABLE_NAME VARCHAR(256) NOT NULL, 
  F_GEOMETRY_COLUMN VARCHAR(256) NOT NULL,
  COORD_DIMENSION INTEGER NOT NULL, 
  SRID INTEGER NOT NULL, 
  TYPE VARCHAR(30) NOT NULL 
)</programlisting>

        <para>The columns are as follows:</para>

        <variablelist>
          <varlistentry>
            <term>F_TABLE_CATALOG, F_TABLE_SCHEMA, F_TABLE_NAME</term>

            <listitem>
              <para>The fully qualified name of the feature table containing
              the geometry column. Note that the terms "catalog" and "schema"
              are Oracle-ish. There is not PostgreSQL analogue of "catalog" so
              that column is left blank -- for "schema" the database name is
              used.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>F_GEOMETRY_COLUMN</term>

            <listitem>
              <para>The name of the geometry column in the feature
              table.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>COORD_DIMENSION</term>

            <listitem>
              <para>The spatial dimension (2 or 3 dimensional) of the
              column.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>SRID</term>

            <listitem>
              <para>The ID of the spatial reference system used for the
              coordinate geometry in this table. It is a foreign key reference
              to the SPATIAL_REF_SYS.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>TYPE</term>

            <listitem>
              <para>The type of the spatial object. To restrict the spatial
              column to a single type, use one of: POINT, LINESTRING, POLYGON,
              MULTPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION.
              For heterogeneous (mixed-type) collections, you can use
              "GEOMETRY" as the type.</para>

              <note>
                <para>This attribute is (probably) not part of the OpenGIS
                specification, but is required for ensuring type
                homogeneity.</para>
              </note>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect2>

      <sect2>
        <title>Creating a Spatial Table</title>

        <para>Creating a table with spatial data is done in two stages:</para>

        <itemizedlist>
          <listitem>
            <para>Create a normal non-spatial table.</para>

            <para>For example: CREATE TABLE ROADS_GEOM ( ID int4, NAME
            varchar(25) )</para>
          </listitem>

          <listitem>
            <para>Add a spatial column to the table using the OpenGIS
            "AddGeometryColumn" function. The syntax is:
            AddGeometryColumn(&lt;db_name&gt;, &lt;table_name&gt;,
            &lt;column_name&gt;, &lt;srid&gt;, &lt;type&gt;,
            &lt;dimension&gt;).</para>

            <para>For example: SELECT AddGeometryColumn('roads_db',
            'roads_geom', 'geom', 423, 'LINESTRING', 2)</para>
          </listitem>
        </itemizedlist>

        <para>Here is an example of SQL used to create a table and add a
        spatial column (assuming the db is 'parks_db' and that an SRID of 128
        exists already):</para>

        <programlisting>CREATE TABLE PARKS ( PARK_ID int4, PARK_NAME varchar(128), PARK_DATE date, PARK_TYPE varchar(2) );
SELECT AddGeometryColumn('parks_db', 'parks', 'park_geom', 128, 'MULTIPOLYGON', 2 );</programlisting>

        <para>Here is another example, using the generic "geometry" type and
        the undefined SRID value of -1:</para>

        <programlisting>CREATE TABLE ROADS ( ROAD_ID int4, ROAD_NAME varchar(128) ); 
SELECT AddGeometryColumn( 'roads_db', 'roads', 'roads_geom', -1, 'GEOMETRY', 3 );</programlisting>
      </sect2>
    </sect1>

    <sect1>
      <title>Loading GIS Data</title>

      <para>Once you have created a spatial table, you are ready to upload GIS
      data to the database. Currently, there are two ways to get data into a
      PostGIS/PostgreSQL database: using formatted SQL statements or using the
      Shape file loader/dumper.</para>

      <sect2>
        <title>Using SQL</title>

        <para>If you can convert your data to a text representation, then
        using formatted SQL might be the easiest way to get your data into
        PostGIS. As with Oracle and other SQL databases, data can be bulk
        loaded by piping a large text file full of SQL "INSERT" statements
        into the SQL terminal monitor.</para>

        <para>A data upload file (<filename>roads.sql</filename> for example)
        might look like this:</para>

        <programlisting>BEGIN;
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (1,GeometryFromText('LINESTRING(191232 243118,191108 243242)',-1),'Jeff Rd'); 
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (2,GeometryFromText('LINESTRING(189141 244158,189265 244817)',-1),'Geordie Rd'); 
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (3,GeometryFromText('LINESTRING(192783 228138,192612 229814)',-1),'Paul St'); 
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (4,GeometryFromText('LINESTRING(189412 252431,189631 259122)',-1),'Graeme Ave'); 
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (5,GeometryFromText('LINESTRING(190131 224148,190871 228134)',-1),'Phil Tce'); 
INSERT INTO ROADS_GEOM (ID,GEOM,NAME ) VALUES (6,GeometryFromText('LINESTRING(198231 263418,198213 268322)',-1),'Dave Cres');
COMMIT;</programlisting>

        <para>The data file can be piped into PostgreSQL very easily using the
        "psql" SQL terminal monitor:</para>

        <programlisting>psql -d [database] -f roads.sql</programlisting>
      </sect2>

      <sect2>
        <title>Using the Loader</title>

        <para>The <filename>shp2pgsql</filename> data loader converts ESRI
        Shape files into SQL suitable for insertion into a PostGIS/PostgreSQL
        database. The loader has several operating modes distinguished by
        command line flags:</para>

        <variablelist>
          <varlistentry>
            <term>-d</term>

            <listitem>
              <para>Drops the database table before creating a new table with
              the data in the Shape file.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-a</term>

            <listitem>
              <para>Appends data from the Shape file into the database table.
              Note that to use this option to load multiple files, the files
              must have the same attributes and same data types.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-c</term>

            <listitem>
              <para>Creates a new table and populates it from the Shape file.
              <emphasis>This is the default mode.</emphasis></para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-D</term>

            <listitem>
              <para>Creates a new table and populates it from the Shape file.
              This uses the PostgreSQL "dump" format for the output data and
              is much faster to load than the default "insert" SQL format. Use
              this for very large data sets.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-s &lt;SRID&gt;</term>

            <listitem>
              <para>Creates and populates the geometry tables with the
              specified SRID.</para>
            </listitem>
          </varlistentry>
        </variablelist>

        <para>An example session using the loader to create an input file and
        uploading it might look like this:</para>

        <programlisting># shp2pgsql shaperoads roadstable roadsdb &gt; roads.sql 
# psql -d roadsdb -f roads.sql</programlisting>

        <para>A conversion and upload can be done all in one step using UNIX
        pipes:</para>

        <programlisting># shp2pgsql shaperoads roadstable roadsdb | psql -d roadsdb</programlisting>
      </sect2>
    </sect1>

    <sect1>
      <title>Retrieving GIS Data</title>

      <para>Data can be extracted from the database using either SQL or the
      Shape file loader/dumper. In the section on SQL we will discuss some of
      the operators available to do comparisons and queries on spatial
      tables.</para>

      <sect2>
        <title>Using SQL</title>

        <para>The most straightforward means of pulling data out of the
        database is to use a SQL select query and dump the resulting columns
        into a parsable text file:</para>

        <programlisting>db=# SELECT id, AsText(geom) AS geom, name FROM ROADS_GEOM; 
id | geom                                    | name 
---+-----------------------------------------+-----------
 1 | LINESTRING(191232 243118,191108 243242) | Jeff Rd  
 2 | LINESTRING(189141 244158,189265 244817) | Geordie Rd 
 3 | LINESTRING(192783 228138,192612 229814) | Paul St 
 4 | LINESTRING(189412 252431,189631 259122) | Graeme Ave 
 5 | LINESTRING(190131 224148,190871 228134) | Phil Tce 
 6 | LINESTRING(198231 263418,198213 268322) | Dave Cres 
 7 | LINESTRING(218421 284121,224123 241231) | Chris Way 
(6 rows)</programlisting>

        <para>However, there will be times when some kind of restriction is
        necessary to cut down the number of fields returned. In the case of
        attribute-based restrictions, just use the same SQL syntax as normal
        with a non-spatial table. In the case of spatial restrictions, the
        following operators are available/useful:</para>

        <variablelist>
          <varlistentry>
            <term>&amp;&amp;</term>

            <listitem>
              <para>This operator tells whether the bounding box of one
              geometry overlaps the bounding box of another.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>~=</term>

            <listitem>
              <para>This operators tests whether two geometries are
              geometrically identical. For example, if 'POLYGON((0 0,1 1,1 0,0
              0))' is the same as 'POLYGON((0 0,1 1,1 0,0 0))' (it is).</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>=</term>

            <listitem>
              <para>This operator is a little more naive, it only tests
              whether the bounding boxes of to geometries are the same.</para>
            </listitem>
          </varlistentry>
        </variablelist>

        <para>Next, you can use these operators in queries. Note that when
        specifying geometries and boxes on the SQL command line, you must
        explicitly turn the string representations into geometries by using
        the "GeometryFromText()" function. So, for example:</para>

        <programlisting>SELECT 
  ID, NAME 
FROM ROADS_GEOM 
WHERE 
  GEOM ~= GeometryFromText('LINESTRING(191232 243118,191108 243242)',-1);</programlisting>

        <para>The above query would return the single record from the
        "ROADS_GEOM" table in which the geometry was equal to that
        value.</para>

        <para>When using the "&amp;&amp;" operator, you can specify either a
        BOX3D as the comparison feature or a GEOMETRY. When you specify a
        GEOMETRY, however, its bounding box will be used for the
        comparison.</para>

        <programlisting>SELECT 
  ID, NAME 
FROM ROADS_GEOM 
WHERE 
  GEOM &amp;&amp; GeometryFromText('POLYGON((191232 243117,191232 243119,191234 243117,191232 243117))',-1);</programlisting>

        <para>The above query will use the bounding box of the polygon for
        comparison purposes.</para>

        <para>The most common spatial query will probably be a "frame-based"
        query, used by client software, like data browsers and web mappers, to
        grab a "map frame" worth of data for display. Using a "BOX3D" object
        for the frame, such a query looks like this:</para>

        <programlisting>SELECT 
  AsText(GEOM) AS GEOM 
FROM ROADS_GEOM 
WHERE 
  GEOM &amp;&amp; GeometryFromText('BOX3D(191232 243117,191232 243119)'::box3d,-1);</programlisting>

        <para>Note the use of the SRID, to specify the projection of the
        BOX3D. The value -1 is used to indicate no specified SRID.</para>
      </sect2>

      <sect2>
        <title>Using the Dumper</title>

        <para>The <filename>pgsql2shp</filename> table dumper connects
        directly to the database and converts a table into a shape file. The
        basic syntax is:</para>

        <programlisting>pgsql2shp [&lt;options&gt;] &lt;database&gt; &lt;table&gt;</programlisting>

        <para>The commandline options are:</para>

        <variablelist>
          <varlistentry>
            <term>-d</term>

            <listitem>
              <para>Write a 3-dimensional shape file. The default is to write
              a 2-dimensional shape file.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-f &lt;filename&gt;</term>

            <listitem>
              <para>Write the output to a particular filename.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-h &lt;host&gt;</term>

            <listitem>
              <para>The database host to connect to.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-p &lt;port&gt;</term>

            <listitem>
              <para>The port to connect to on the database host.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-P &lt;password&gt;</term>

            <listitem>
              <para>The password to use when connecting to the
              database.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-u &lt;user&gt;</term>

            <listitem>
              <para>The username to use when connecting to the
              database.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>-g &lt;geometry column&gt;</term>

            <listitem>
              <para>In the case of tables with multiple geometry columns, the
              geometry column to use when writing the shape file.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect2>
    </sect1>

    <sect1>
      <title>Building Indexes</title>

      <para>Indexes are what make using a spatial database for large databases
      possible. Without indexing, any search for a feature would require a
      "sequential scan" of every record in the database. Indexing speeds up
      searching by organizing the data into a search tree which can be quickly
      traversed to find a particular record. PostgreSQL supports three kinds
      of indexes by default: B-Tree indexes, R-Tree indexes, and GiST
      indexes.</para>

      <itemizedlist>
        <listitem>
          <para>B-Trees are used for data which can be sorted along one axis;
          for example, numbers, letters, dates. GIS data cannot be rationally
          sorted along one axis (which is greater, (0,0) or (0,1) or (1,0)?)
          so B-Tree indexing is of no use for us.</para>
        </listitem>

        <listitem>
          <para>R-Trees break up data into rectangles, and sub-rectangles, and
          sub-sub rectangles, etc. R-Trees are used by some spatial databases
          to index GIS data, but the PostgreSQL R-Tree implementation is not
          as robust as the GiST implementation.</para>
        </listitem>

        <listitem>
          <para>GiST (Generalized Search Trees) indexes break up data into
          "things to one side", "things which overlap", "things which are
          inside" and can be used on a wide range of data-types, including GIS
          data. PostGIS uses an R-Tree index implemented on top of GiST to
          index GIS data.</para>
        </listitem>
      </itemizedlist>

      <sect2>
        <title>GiST Indexes</title>

        <para>GiST stands for "Generalized Search Tree" and is a generalized
        form of indexing. In addition to GIS indexing, GiST is used to speed
        up searches on all kinds of irregular data structures (integer arrays,
        spectral data, etc) which are not amenable to normal B-Tree
        indexing.</para>

        <para>Once a GIS data table exceeds a few thousand rows, you will want
        to build an index to speed up spatial searches of the data (unless all
        your searches are based on attributes, in which case you'll want to
        build a normal index on the attribute fields).</para>

        <para>The syntax for building a GiST index on a "geometry" column is
        as follows:</para>

        <para><programlisting>CREATE INDEX [indexname] ON [tablename] 
  USING GIST ( [geometryfield] GIST_GEOMETRY_OPS ); </programlisting></para>

        <para>Building a spatial index is a computationally intensive
        exercise: on tables of around 1 million rows, on a 300MHz Solaris
        machine, we have found building a GiST index takes about 1 hour. After
        building an index, it is important to force PostgreSQL to collect
        table statistics, which are used to optimize query plans:</para>

        <para><programlisting>VACUUM ANALYZE [table_name] [column_name];

-- This is only needed for PostgreSQL 7.4 installations and below
SELECT UPDATE_GEOMETRY_STATS([table_name], [column_name]);</programlisting></para>

        <para>GiST indexes have two advantages over R-Tree indexes in
        PostgreSQL. Firstly, GiST indexes are "null safe", meaning they can
        index columns which include null values. Secondly, GiST indexes
        support the concept of "lossiness" which is important when dealing
        with GIS objects larger than the PostgreSQL 8K page size. Lossiness
        allows PostgreSQL to store only the "important" part of an object in
        an index -- in the case of GIS objects, just the bounding box. GIS
        objects larger than 8K will cause R-Tree indexes to fail in the
        process of being built.</para>
      </sect2>

      <sect2>
        <title>Using Indexes</title>

        <para>Ordinarily, indexes invisibly speed up data access: once the
        index is built, the query planner transparently decides when to use
        index information to speed up a query plan. Unfortunately, the
        PostgreSQL query planner does not optimize the use of GiST indexes
        well, so sometimes searches which should use a spatial index instead
        default to a sequence scan of the whole table.</para>

        <para>If you find your spatial indexes are not being used (or your
        attribute indexes, for that matter) there are a couple things you can
        do:</para>

        <itemizedlist>
          <listitem>
            <para>Firstly, make sure statistics are gathered about the number
            and distributions of values in a table, to provide the query
            planner with better information to make decisions around index
            usage. For PostgreSQL 7.4 installations and below this is done by
            running "update_geometry_stats([table_name, column_name])"
            (compute distribution) and "VACUUM ANALYZE [table_name]
            [column_name]" (compute number of values). Starting with
            PostgreSQL 8.0 running "VACUUM ANALYZE" will do both operations.
            You should regularly vacuum your databases anyways -- many
            PostgreSQL DBAs have "VACUUM" run as an off-peak cron job on a
            regular basis.</para>
          </listitem>

          <listitem>
            <para>If vacuuming does not work, you can force the planner to use
            the index information by using the "SET ENABLE_SEQSCAN=OFF"
            command. You should only use this command sparingly, and only on
            spatially indexed queries: generally speaking, the planner knows
            better than you do about when to use normal B-Tree indexes. Once
            you have run your query, you should consider setting
            "ENABLE_SEQSCAN" back on, so that other queries will utilize the
            planner as normal.</para>

            <note>
              <para>As of version 0.6, it should not be necessary to force the
              planner to use the index with "ENABLE_SEQSCAN".</para>
            </note>
          </listitem>
        </itemizedlist>
      </sect2>
    </sect1>

    <sect1>
      <title>Complex Queries</title>

      <para>The <emphasis>raison d'etre</emphasis> of spatial database
      functionality is performing queries inside the database which would
      ordinarily require desktop GIS functionality. Using PostGIS effectively
      requires knowing what spatial functions are available, and ensuring that
      appropriate indexes are in place to provide good performance.</para>

      <sect2>
        <title>Taking Advantage of Indexes</title>

        <para>When constructing a query it is important to remember that only
        the bounding-box-based operators such as &amp;&amp; can take advatage
        of the GiST spatial index. Functions such as
        <varname>distance()</varname> cannot use the index to optimize their
        operation. For example, the following query would be quite slow on a
        large table:</para>

        <programlisting>SELECT the_geom FROM geom_table
WHERE distance( the_geom, GeometryFromText( 'POINT(100000 200000)', -1 ) ) &lt; 100</programlisting>

        <para>This query is selecting all the geometries in geom_table which
        are within 100 units of the point (100000, 200000). It will be slow
        because it is calculating the distance between each point in the table
        and our specified point, ie. one <varname>distance()</varname>
        calculation for each row in the table. We can avoid this by using the
        &amp;&amp; operator to reduce the number of distance calculations
        required:</para>

        <programlisting>SELECT the_geom FROM geom_table
WHERE the_geom &amp;&amp; 'BOX3D(90900 190900, 100100 200100)'::box3d
  AND distance( the_geom, GeometryFromText( 'POINT(100000 200000)', -1 ) ) &lt; 100</programlisting>

        <para>This query selects the same geometries, but it does it in a more
        efficient way. Assuming there is a GiST index on the_geom, the query
        planner will recognize that it can use the index to reduce the number
        of rows before calculating the result of the
        <varname>distance()</varname> function. Notice that the
        <varname>BOX3D</varname> geometry which is used in the &amp;&amp;
        operation is a 200 unit square box centered on the original point -
        this is our "query box". The &amp;&amp; operator uses the index to
        quickly reduce the result set down to only those geometries which have
        bounding boxes that overlap the "query box". Assuming that our query
        box is much smaller than the extents of the entire geometry table,
        this will drastically reduce the number of distance calculations that
        need to be done.</para>
      </sect2>
    </sect1>

    <sect1>
      <title>Using Mapserver</title>

      <para>The Minnesota Mapserver is an internet web-mapping server which
      conforms to the OpenGIS Web Mapping Server specification.</para>

      <itemizedlist>
        <listitem>
          <para>The Mapserver homepage is at <ulink
          url="http://mapserver.gis.umn.edu">http://mapserver.gis.umn.edu</ulink>.</para>
        </listitem>

        <listitem>
          <para>The OpenGIS Web Map Specification is at <ulink
          url="http://www.opengis.org/techno/specs/01-047r2.pdf">http://www.opengis.org/techno/specs/01-047r2.pdf</ulink>.</para>
        </listitem>
      </itemizedlist>

      <sect2>
        <title>Basic Usage</title>

        <para>To use PostGIS with Mapserver, you will need to know about how
        to configure Mapserver, which is beyond the scope of this
        documentation. This section will cover specific PostGIS issues and
        configuration details.</para>

        <para>To use PostGIS with Mapserver, you will need:</para>

        <itemizedlist>
          <listitem>
            <para>Version 0.6 or newer of PostGIS.</para>
          </listitem>

          <listitem>
            <para>Version 3.5 or newer of Mapserver.</para>
          </listitem>
        </itemizedlist>

        <para>Mapserver accesses PostGIS/PostgreSQL data like any other
        PostgreSQL client -- using <filename>libpq</filename>. This means that
        Mapserver can be installed on any machine with network access to the
        PostGIS server, as long as the system has the
        <filename>libpq</filename> PostgreSQL client libraries.</para>

        <orderedlist>
          <listitem>
            <para>Compile and install Mapserver, with whatever options you
            desire, including the "--with-postgis" configuration
            option.</para>
          </listitem>

          <listitem>
            <para>In your Mapserver map file, add a PostGIS layer. For
            example:</para>

            <programlisting>LAYER
  CONNECTIONTYPE postgis
  NAME "widehighways"
  # Connect to a remote spatial database
  CONNECTION "user=dbuser dbname=gisdatabase host=bigserver"
  # Get the lines from the 'geom' column of the 'roads' table
  DATA "geom from roads"
  STATUS ON
  TYPE LINE
  # Of the lines in the extents, only render the wide highways
  FILTER "type = 'highway' and numlanes &gt;= 4"
  CLASS
    # Make the superhighways brighter and 2 pixels wide
    EXPRESSION ([numlanes] &gt;= 6)
    COLOR 255 22 22      
    SYMBOL "solid"
    SIZE 2
  END
  CLASS
    # All the rest are darker and only 1 pixel wide
    EXPRESSION ([numlanes] &lt; 6)
    COLOR 205 92 82      
  END
END</programlisting>

            <para>In the example above, the PostGIS-specific directives are as
            follows:</para>

            <variablelist>
              <varlistentry>
                <term>CONNECTIONTYPE</term>

                <listitem>
                  <para>For PostGIS layers, this is always "postgis".</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>CONNECTION</term>

                <listitem>
                  <para>The database connection is governed by the a
                  'connection string' which is a standard set of keys and
                  values like this (with the default values in
                  &lt;&gt;):</para>

                  <para>user=&lt;username&gt; password=&lt;password&gt;
                  dbname=&lt;username&gt; hostname=&lt;server&gt;
                  port=&lt;5432&gt;</para>

                  <para>An empty connection string is still valid, and any of
                  the key/value pairs can be omitted. At a minimum you will
                  generally supply the database name and username to connect
                  with.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>DATA</term>

                <listitem>
                  <para>The form of this parameter is "&lt;column&gt; from
                  &lt;tablename&gt;" where the column is the spatial column to
                  be rendered to the map.</para>
                </listitem>
              </varlistentry>

              <varlistentry>
                <term>FILTER</term>

                <listitem>
                  <para>The filter must be a valid SQL string corresponding to
                  the logic normally following the "WHERE" keyword in a SQL
                  query. So, for example, to render only roads with 6 or more
                  lanes, use a filter of "num_lanes &gt;= 6".</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>

          <listitem>
            <para>In your spatial database, ensure you have spatial (GiST)
            indexes built for any the layers you will be drawing.</para>

            <programlisting>CREATE INDEX [indexname]
  ON [tablename] 
  USING GIST ( [geometrycolumn] GIST_GEOMETRY_OPS );</programlisting>
          </listitem>

          <listitem>
            <para>If you will be querying your layers using Mapserver you will
            also need an "oid index".</para>

            <para>Mapserver requires unique identifiers for each spatial
            record when doing queries, and the PostGIS module of Mapserver
            uses the PostgreSQL <varname>oid</varname> value to provide these
            unique identifiers. A side-effect of this is that in order to do
            fast random access of records during queries, an index on the
            <varname>oid</varname> is needed.</para>

            <para>To build an "oid index", use the following SQL:</para>

            <programlisting>CREATE INDEX [indexname] ON [tablename] ( oid );</programlisting>
          </listitem>
        </orderedlist>
      </sect2>

      <sect2>
        <title>Advanced Usage</title>

        <para>The <varname>USING</varname> pseudo-SQL clause is used to add
        some information to help mapserver understand the results of more
        complex queries. More specifically, when either a view or a subselect
        is used as the source table (the thing to the right of "FROM" in a
        <varname>DATA</varname> definition) it is more difficult for mapserver
        to automatically determine a unique identifier for each row and also
        the SRID for the table. The <varname>USING</varname> clause can
        provide mapserver with these two pieces of information as
        follows:</para>

        <programlisting>DATA "the_geom FROM (SELECT table1.the_geom AS the_geom, table1.oid AS oid, table2.data AS data
 FROM table1 LEFT JOIN table2 ON table1.id = table2.id) AS new_table USING UNIQUE oid USING SRID=-1"</programlisting>

        <variablelist>
          <varlistentry>
            <term>USING UNIQUE &lt;uniqueid&gt;</term>

            <listitem>
              <para>Mapserver requires a unique id for each row in order to
              identify the row when doing map queries. Normally, it would use
              the oid as the unique identifier, but views and subselects don't
              automatically have an oid column. If you want to use Mapserver's
              query functionality, you need to add a unique column to your
              view or subselect, and declare it with <varname>USING
              UNIQUE</varname>. For example, you could explicitly select one
              of the table's oid values for this purpose, or any other column
              which is guaranteed to be unique for the result set.</para>

              <para>The <varname>USING</varname> statement can also be useful
              even for simple <varname>DATA</varname> statements, if you are
              doing map queries. It was previously recommended to add an index
              on the oid column of tables used in query-able layers, in order
              to speed up the performance of map queries. However, with the
              <varname>USING</varname> clause, it is possible to tell
              mapserver to use your table's primary key as the identifier for
              map queries, and then it is no longer necessary to have an
              additional index.</para>

              <note>
                <para>"Querying a Map" is the action of clicking on a map to
                ask for information about the map features in that location.
                Don't confuse "map queries" with the SQL query in a
                <varname>DATA</varname> definition.</para>
              </note>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>USING SRID=&lt;srid&gt;</term>

            <listitem>
              <para>PostGIS needs to know which spatial referencing system is
              being used by the geometries in order to return the correct data
              back to mapserver. Normally it is possible to find this
              information in the "geometry_columns" table in the PostGIS
              database, however, this is not possible for tables which are
              created on the fly such as subselects and views. So the
              <varname>USING SRID=</varname> option allows the correct SRID to
              be specified in the <varname>DATA</varname> definition.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect2>

      <sect2>
        <title>Examples</title>

        <para>Lets start with a simple example and work our way up. Consider
        the following Mapserver layer definition:</para>

        <programlisting>LAYER
 CONNECTIONTYPE postgis
 NAME "roads"
 CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
 DATA "the_geom FROM roads"
 STATUS ON
 TYPE LINE
 CLASS
  COLOR 0 0 0
 END
END</programlisting>

        <para>This layer will display all the road geometries in the roads
        table as black lines.</para>

        <para>Now lets say we want to show only the highways until we get
        zoomed in to at least a 1:100000 scale - the next two layers will
        acheive this effect:</para>

        <programlisting>LAYER
 CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
 DATA "the_geom FROM roads"
 MINSCALE 100000
 STATUS ON
 TYPE LINE
 FILTER "road_type = 'highway'"
 CLASS
  COLOR 0 0 0
 END
END

LAYER
 CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
 DATA "the_geom FROM roads"
 MAXSCALE 100000
 STATUS ON
 TYPE LINE
 CLASSITEM road_type
 CLASS
  EXPRESSION "highway"
  SIZE 2
  COLOR 255 0 0
 END
 CLASS
  COLOR 0 0 0
 END
END</programlisting>

        <para>The first layer is used when the scale is greater than 1:100000,
        and displays only the roads of type "highway" as black lines. The
        <varname>FILTER</varname> option causes only roads of type "highway"
        to be displayed.</para>

        <para>The second layer is used when the scale is less than 1:100000,
        and will display highways as double-thick red lines, and other roads
        as regular black lines.</para>

        <para>So, we have done a couple of interesting things using only
        mapserver functionality, but our <varname>DATA</varname> SQL statement
        has remained simple. Suppose that the name of the road is stored in
        another table (for whatever reason) and we need to do a join to get it
        and label our roads.</para>

        <programlisting>LAYER
 CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
 DATA "the_geom FROM (SELECT roads.oid AS oid, roads.the_geom AS the_geom, road_names.name as name
   FROM roads LEFT JOIN road_names ON roads.road_name_id = road_names.road_name_id) AS named_roads
   USING UNIQUE oid USING SRID=-1"
 MAXSCALE 20000
 STATUS ON
 TYPE ANNOTATION
 LABELITEM name
 CLASS
  LABEL
   ANGLE auto
   SIZE 8
   COLOR 0 192 0
   TYPE truetype
   FONT arial
  END
 END
END</programlisting>

        <para>This annotation layer adds green labels to all the roads when
        the scale gets down to 1:20000 or less. It also demonstrates how to
        use an SQL join in a <varname>DATA</varname> definition.</para>
      </sect2>
    </sect1>

    <sect1>
      <title>Java Clients (JDBC)</title>

      <para>Java clients can access PostGIS "geometry" objects in the
      PostgreSQL database either directly as text representations or using the
      JDBC extension objects bundled with PostGIS. In order to use the
      extension objects, the "postgis.jar" file must be in your CLASSPATH
      along with the "postgresql.jar" JDBC driver package.</para>

      <programlisting>import java.sql.*; 
import java.util.*; 
import java.lang.*; 
import org.postgis.*; 

public class JavaGIS { 
  public static void main(String[] args) 
  { 
    java.sql.Connection conn; 
    try 
    { 
      /* 
      * Load the JDBC driver and establish a connection. 
      */  
      Class.forName("org.postgresql.Driver"); 
      String url = "jdbc:postgresql://localhost:5432/database"; 
      conn = DriverManager.getConnection(url, "postgres", ""); 
    
      /* 
      * Add the geometry types to the connection. Note that you 
      * must cast the connection to the pgsql-specific connection * implementation before calling the addDataType() method. 
      */
      ((org.postgresql.Connection)conn).addDataType("geometry","org.postgis.PGgeometry");
      ((org.postgresql.Connection)conn).addDataType("box3d","org.postgis.PGbox3d");

      /* 
      * Create a statement and execute a select query. 
      */ 
      Statement s = conn.createStatement(); 
      ResultSet r = s.executeQuery("select AsText(geom) as geom,id from geomtable"); 
      while( r.next() ) 
      { 
        /* 
        * Retrieve the geometry as an object then cast it to the geometry type. 
        * Print things out. 
        */ 
        PGgeometry geom = (PGgeometry)r.getObject(1); 
        int id = r.getInt(2);
        System.out.println("Row " + id + ":"); 
        System.out.println(geom.toString()); 
      }
      s.close(); 
      conn.close(); 
    } 
    catch( Exception e ) 
    { 
      e.printStackTrace(); 
    }  
  }
}</programlisting>

      <para>The "PGgeometry" object is a wrapper object which contains a
      specific topological geometry object (subclasses of the abstract class
      "Geometry") depending on the type: Point, LineString, Polygon,
      MultiPoint, MultiLineString, MultiPolygon.</para>

      <programlisting>PGgeometry geom = (PGgeometry)r.getObject(1); 
if( geom.getType() = Geometry.POLYGON ) 
{ 
  Polygon pl = (Polygon)geom.getGeometry();
  for( int r = 0; r &lt; pl.numRings(); r++ ) 
  { 
    LinearRing rng = pl.getRing(r);
    System.out.println("Ring: " + r); 
    for( int p = 0; p &lt; rng.numPoints(); p++ ) 
    { 
      Point pt = rng.getPoint(p); 
      System.out.println("Point: " + p);
      System.out.println(pt.toString()); 
    } 
  } 
}</programlisting>

      <para>The JavaDoc for the extension objects provides a reference for the
      various data accessor functions in the geometric objects.</para>
    </sect1>

    <sect1>
      <title>C Clients (libpq)</title>

      <para>...</para>

      <sect2>
        <title>Text Cursors</title>

        <para>...</para>
      </sect2>

      <sect2>
        <title>Binary Cursors</title>

        <para>...</para>
      </sect2>
    </sect1>
  </chapter>

  <chapter>
    <title>PostGIS Reference</title>

    <para>The functions given below are the ones which a user of PostGIS is
    likely to need. There are other functions which are required support
    functions to the PostGIS objects which are not of use to a general
    user.</para>

    <sect1>
      <title>OpenGIS Functions</title>

      <variablelist>
        <varlistentry>
          <term>AddGeometryColumn(varchar, varchar, varchar, integer, varchar,
          integer)</term>

          <listitem>
            <para>Syntax: AddGeometryColumn(&lt;schema_name&gt;,
            &lt;table_name&gt;, &lt;column_name&gt;, &lt;srid&gt;,
            &lt;type&gt;, &lt;dimension&gt;). Adds a geometry column to an
            existing table of attributes. The <varname>schema_name</varname>
            is the name of the table schema (unused for pre-schema PostgreSQL
            installations). The <varname>srid</varname> must be an integer
            value reference to an entry in the SPATIAL_REF_SYS table. The
            <varname>type</varname> must be an uppercase string corresponding
            to the geometry type, eg, 'POLYGON' or 'MULTILINESTRING'.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>DropGeometryColumn(varchar, varchar, varchar)</term>

          <listitem>
            <para>Syntax: DropGeometryColumn(&lt;schema_name&gt;,
            &lt;table_name&gt;, &lt;column_name&gt;). Remove a geometry column
            from a spatial table. Note that schema_name will need to match the
            f_schema_name field of the table's row in the geometry_columns
            table.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>AsBinary(geometry)</term>

          <listitem>
            <para>Returns the geometry in the OGC "well-known-binary" format,
            using the endian encoding of the server on which the database is
            running. This is useful in binary cursors to pull data out of the
            database without converting it to a string representation.</para>

            <para>OGC SPEC s2.1.1.1 - also see
            asBinary(&lt;geometry&gt;,'XDR') and
            asBinary(&lt;geometry&gt;,'NDR')</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Dimension(geometry)</term>

          <listitem>
            <para>The inherent dimension of this Geometry object, which must
            be less than or equal to the coordinate dimension. OGC SPEC
            s2.1.1.1 - returns 0 for points, 1 for lines, 2 for polygons, and
            the largest dimension of the components of a
            GEOMETRYCOLLECTION.</para>

            <programlisting>select dimension('GEOMETRYCOLLECTION(LINESTRING(1 1,0 0),POINT(0 0)'); 
dimension 
-----------
1</programlisting>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>isEmpty(geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is the empty geometry . If
            true, then this Geometry represents the empty point set - i.e.
            GEOMETRYCOLLECTION(EMPTY).</para>

            <para>OGC SPEC s2.1.1.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>isSimple(geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry has no anomalous geometric
            points, such as self intersection or self tangency.</para>

            <para>Performed by the GEOS module</para>

            <para>OGC SPEC s2.1.1.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>boundary(geometry)</term>

          <listitem>
            <para>Returns the closure of the combinatorial boundary of this
            Geometry. The combinatorial boundary is defined as described in
            section 3.12.3.2 of the OGC SPEC. Because the result of this
            function is a closure, and hence topologically closed, the
            resulting boundary can be represented using representational
            geometry primitives as discussed in the OGC SPEC, section
            3.12.2.</para>

            <para>Performed by the GEOS module</para>

            <para>OGC SPEC s2.1.1.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>equals(geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially equal" to
            anotherGeometry. Use this for a 'better' answer than '='. equals
            ('LINESTRING(0 0, 10 10)','LINESTRING(0 0, 5 5, 10 10)') is
            true.</para>

            <para>Performed by the GEOS module</para>

            <para>OGC SPEC s2.1.1.2</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>disjoint(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially disjoint"
            from anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 //s2.1.13.3 - a.Relate(b,
            'FF*FF****')</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>intersects(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry "spatially intersects"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 //s2.1.13.3 - Intersects(g1, g2 ) --&gt;
            Not (Disjoint(g1, g2 ))</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>touches(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry "spatially touches"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3- a.Touches(b) -&gt; (I(a)
            intersection I(b) = {empty set} ) and (a intersection b) not
            empty</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>crosses(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry "spatially crosses"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3 - a.Relate(b,
            'T*T******')</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>within(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially within"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3 - a.Relate(b,
            'T*F**F***')</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>overlaps(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially overlapping"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>contains(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially contains"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3 - same as
            within(geometry,geometry)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>intersects(geometry,geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is "spatially intersects"
            anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3 - NOT
            disjoint(geometry,geometry)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>relate(geometry,geometry, intersectionPatternMatrix)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Geometry is spatially related to
            anotherGeometry, by testing for intersections between the
            Interior, Boundary and Exterior of the two geometries as specified
            by the values in the intersectionPatternMatrix.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is the "allowable" version that returns a
            boolean, not an integer.</para>

            <para>OGC SPEC s2.1.1.2 // s2.1.13.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>relate(geometry,geometry)</term>

          <listitem>
            <para>returns the DE-9IM (dimensionally extended nine-intersection
            matrix)</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>not in OGC spec, but implied. see s2.1.13.2</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>buffer(geometry,double)</term>

          <listitem>
            <para>Returns a geometry that represents all points whose distance
            from this Geometry is less than or equal to distance. Calculations
            are in the Spatial Reference System of this Geometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>convexhull(geometry)</term>

          <listitem>
            <para>Returns a geometry that represents the convex hull of this
            Geometry.</para>

            <para>Performed by the GEOS module</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>intersection(geometry,geometry)</term>

          <listitem>
            <para>Returns a geometry that represents the point set
            intersection of this Geometry with anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <!-- Should this be documented? -->

        <varlistentry>
          <term>GeomUnion(geometry,geometry)</term>

          <listitem>
            <para>Returns a geometry that represents the point set union of
            this Geometry with anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>NOTE: this is renamed from "union" because union is an SQL
            reserved word</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomUnion(geometry set)</term>

          <listitem>
            <para>Returns a geometry that represents the point set union of
            this all Geometries in given set.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection in the argument
            set</para>

            <para>Not explicitly defined in OGC SPEC</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>memGeomUnion(geometry set)</term>

          <listitem>
            <para>Same as the above, only memory-friendly (uses less memory
            and more processor time).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>symdifference(geometry,geometry)</term>

          <listitem>
            <para>Returns a geometry that represents the point set symmetric
            difference of this Geometry with anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>difference(geometry,geometry)</term>

          <listitem>
            <para>Returns a geometry that represents the point set symmetric
            difference of this Geometry with anotherGeometry.</para>

            <para>Performed by the GEOS module</para>

            <para>Do not call with a GeometryCollection as an argument</para>

            <para>OGC SPEC s2.1.1.3</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Envelope(geometry)</term>

          <listitem>
            <para>Returns a POLYGON representing the bounding box of the
            geometry.</para>

            <para>OGC SPEC s2.1.1.1 - The minimum bounding box for this
            Geometry, returned as a Geometry. The polygon is defined by the
            corner points of the bounding box ((MINX, MINY), (MAXX, MINY),
            (MAXX, MAXY), (MINX, MAXY), (MINX, MINY)).</para>

            <para>NOTE:PostGIS will add a Zmin/Zmax coordinate as well.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeometryType(geometry)</term>

          <listitem>
            <para>Returns the type of the geometry as a string. Eg:
            'LINESTRING', 'POLYGON', 'MULTIPOINT', etc.</para>

            <para>OGC SPEC s2.1.1.1 - Returns the name of the instantiable
            subtype of Geometry of which this Geometry instance is a member.
            The name of the instantiable subtype of Geometry is returned as a
            string.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>X(geometry)</term>

          <listitem>
            <para>Find and return the X coordinate of the first point in the
            geometry. Return NULL if there is no point in the geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Y(geometry)</term>

          <listitem>
            <para>Find and return the Y coordinate of the first point in the
            geometry. Return NULL if there is no point in the geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Z(geometry)</term>

          <listitem>
            <para>Find and return the Z coordinate of the first point in the
            geometry. Return NULL if there is no point in the geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>NumPoints(geometry)</term>

          <listitem>
            <para>Find and return the number of points in the first linestring
            in the geometry. Return NULL if there is no linestring in the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PointN(geometry,integer)</term>

          <listitem>
            <para>Return the N'th point in the first linestring in the
            geometry. Return NULL if there is no linestring in the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>ExteriorRing(geometry)</term>

          <listitem>
            <para>Return the exterior ring of the first polygon in the
            geometry. Return NULL if there is no polygon in the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>NumInteriorRings(geometry)</term>

          <listitem>
            <para>Return the number of interior rings of the first polygon in
            the geometry. Return NULL if there is no polygon in the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>InteriorRingN(geometry,integer)</term>

          <listitem>
            <para>Return the N'th interior ring of the first polygon in the
            geometry. Return NULL if there is no polygon in the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>IsClosed(geometry)</term>

          <listitem>
            <para>Returns true of the geometry start and end points are
            coincident.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>IsRing(geometry)</term>

          <listitem>
            <para>Returns 1 (TRUE) if this Curve is closed (StartPoint ( ) =
            EndPoint ( )) and this Curve is simple (does not pass through the
            same point more than once).</para>

            <para>performed by GEOS</para>

            <para>OGC spec 2.1.5.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>NumGeometries(geometry)</term>

          <listitem>
            <para>If geometry is a GEOMETRYCOLLECTION (or MULTI*) return the
            number of geometries, otherwise return NULL.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeometryN(geometry,int)</term>

          <listitem>
            <para>Return the N'th geometry if the geometry is a
            GEOMETRYCOLLECTION, MULTIPOINT, MULTILINESTRING or MULTIPOLYGON.
            Otherwise, return NULL.</para>

            <para>1 is 1st geometry</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Distance(geometry,geometry)</term>

          <listitem>
            <para>Return the cartesian distance between two geometries in
            projected units.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>AsText(geometry)</term>

          <listitem>
            <para>Return the Well-Known Text representation of the geometry.
            For example: POLYGON(0 0,0 1,1 1,1 0,0 0)</para>

            <para>OGC SPEC s2.1.1.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>SRID(geometry)</term>

          <listitem>
            <para>Returns the integer SRID number of the spatial reference
            system of the geometry.</para>

            <para>OGC SPEC s2.1.1.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeometryFromText(varchar, integer)</term>

          <listitem>
            <para>Syntax: GeometryFromText(&lt;geometry&gt;,&lt;SRID&gt;)
            Convert a Well-Known Text representation of a geometry into a
            geometry object.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomFromText(varchar, integer)</term>

          <listitem>
            <para>As above. A synonym for GeometryFromText.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>SetSRID(geometry)</term>

          <listitem>
            <para>Set the SRID on a geometry to a particular integer value.
            Useful in constructing bounding boxes for queries.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>EndPoint(geometry)</term>

          <listitem>
            <para>Returns the last point of the geometry as a point.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>StartPoint(geometry)</term>

          <listitem>
            <para>Returns the first point of the geometry as a point.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>Centroid(geometry)</term>

          <listitem>
            <para>Returns the centroid of the geometry as a point.</para>

            <para>Computation will be more accurate if performed by the GEOS
            module (enabled at compile time).</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect1>

    <sect1>
      <title>Other Functions</title>

      <variablelist>
        <varlistentry>
          <term>DropGeometryTable([&lt;schema_name&gt;],
          &lt;table_name&gt;)</term>

          <listitem>
            <para>Drops a table and all its references in geometry_columns.
            Note: uses current_schema() on schema-aware pgsql installations if
            schema is not provided.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>update_geometry_stats([&lt;table_name&gt;,
          &lt;column_name&gt;])</term>

          <listitem>
            <para>Update statistics about spatial tables for use by the query
            planner. You will also need to run "VACUUM ANALYZE [table_name]
            [column_name]" for the statistics gathering process to be
            complete. NOTE: starting with PostgreSQL 8.0 statistics gathering
            is automatically performed running "VACUUM ANALYZE".</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A &amp;&lt; B</term>

          <listitem>
            <para>The "&amp;&lt;" operator returns true if A's bounding box
            overlaps or is to the left of B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A &amp;&gt; B</term>

          <listitem>
            <para>The "&amp;&gt;" operator returns true if A's bounding box
            overlaps or is to the right of B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A &lt;&lt; B</term>

          <listitem>
            <para>The "&lt;&lt;" operator returns true if A's bounding box is
            strictly to the left of B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A &gt;&gt; B</term>

          <listitem>
            <para>The "&gt;&gt;" operator returns true if A's bounding box is
            strictly to the right of B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A ~= B</term>

          <listitem>
            <para>The "~=" operator is the "same as" operator. It tests actual
            geometric equality of two features. So if A and B are the same
            feature, vertex-by-vertex, the operator returns true.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A @ B</term>

          <listitem>
            <para>The "@" operator returns true if A's bounding box is
            completely contained by B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A ~ B</term>

          <listitem>
            <para>The "~" operator returns true if A's bounding box completely
            contains B's bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>A &amp;&amp; B</term>

          <listitem>
            <para>The "&amp;&amp;" operator is the "overlaps" operator. If A's
            bounding boux overlaps B's bounding box the operator returns
            true.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>area2d(geometry)</term>

          <listitem>
            <para>Returns the area of the geometry if it is a polygon or
            multi-polygon.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>area(geometry)</term>

          <listitem>
            <para>Returns the area of the geometry if it is a polygon or
            multi-polygon. (same as area2(&lt;polygon|multipolygon&gt;)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>asbinary(geometry,'NDR')</term>

          <listitem>
            <para>Returns the geometry in the OGC "well-known-binary" format,
            using little-endian encoding. This is useful in binary cursors to
            pull data out of the database without converting it to a string
            representation.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>isvalid(geometry)</term>

          <listitem>
            <para>returns true if this geometry is valid.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>asbinary(geometry,'XDR')</term>

          <listitem>
            <para>Returns the geometry in the OGC "well-known-binary" format,
            using big-endian encoding. This is useful in binary cursors to
            pull data out of the database without converting it to a string
            representation.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeometryFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PointFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a Point</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>LineFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a Line</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>LinestringFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>from the conformance suite</para>

            <para>Throws an error if the WKT is not a Line</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PolyFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a Polygon</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PolygonFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>from the conformance suite</para>

            <para>Throws an error if the WKT is not a Polygon</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MPointFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a MULTIPOINT</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MLineFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a MULTILINESTRING</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MPolyFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a MULTIPOLYGON</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomCollFromText(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKT with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>

            <para>Throws an error if the WKT is not a
            GEOMETRYCOLLECTION</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.6.2 - option SRID is from the conformance
            suite</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PointFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a POINT</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>LineFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a LINESTRING</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>LinestringFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>from the conformance suite</para>

            <para>throws an error if WKB is not a LINESTRING</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PolyFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a POLYGON</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PolygonFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>from the conformance suite</para>

            <para>throws an error if WKB is not a POLYGON</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MPointFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a MULTIPOINT</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MLineFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a MULTILINESTRING</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MPolyFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a MULTIPOLYGON</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>GeomCollFromWKB(text,[&lt;srid&gt;])</term>

          <listitem>
            <para>Makes a Geometry from WKB with the given SRID. If SRID is
            not give, it defaults to -1.</para>

            <para>OGC SPEC 3.2.7.2 - option SRID is from the conformance
            suite</para>

            <para>throws an error if WKB is not a GEOMETRYCOLLECTION</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>PointOnSurface(geometry)</term>

          <listitem>
            <para>Return a Point guaranteed to lie on the surface</para>

            <para>Implemented using GEOS</para>

            <para>OGC SPEC 3.2.14.2 and 3.2.18.2 -</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>box3d(geometry)</term>

          <listitem>
            <para>Returns a BOX3D representing the maximum extents of the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>expand(geometry, float)</term>

          <listitem>
            <para>This function returns a bounding box expanded in all
            directions from the bounding box of the input geometry, by an
            amount specified in the second argument. Very useful for
            distance() queries, to add an index filter to the query.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>collect(geometry set)</term>

          <listitem>
            <para>This function returns a GEOMETRYCOLLECTION object from a set
            of geometries. The collect() function is an "aggregate" function
            in the terminology of PostgreSQL. That means that it operators on
            lists of data, in the same way the sum() and mean() functions do.
            For example, "SELECT COLLECT(GEOM) FROM GEOMTABLE GROUP BY
            ATTRCOLUMN" will return a separate GEOMETRYCOLLECTION for each
            distinct value of ATTRCOLUMN.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>memcollect(geometry set)</term>

          <listitem>
            <para>This does the the same of collect(geometry), only more
            memory-friendly (uses less memory and more processor time).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>distance_spheroid(point, point, spheroid)</term>

          <listitem>
            <para>Returns linear distance between two lat/lon points given a
            particular spheroid. See the explanation of spheroids given for
            <link linkend="length_spheroid">length_spheroid()</link>.
            Currently only implemented for points.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>extent(geometry set)</term>

          <listitem>
            <para>The extent() function is an "aggregate" function in the
            terminology of PostgreSQL. That means that it operators on lists
            of data, in the same way the sum() and mean() functions do. For
            example, "SELECT EXTENT(GEOM) FROM GEOMTABLE" will return a BOX3D
            giving the maximum extend of all features in the table. Similarly,
            "SELECT EXTENT(GEOM) FROM GEOMTABLE GROUP BY CATEGORY" will return
            one extent result for each category.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>find_srid(varchar,varchar,varchar)</term>

          <listitem>
            <para>The syntax is find_srid(&lt;db/schema&gt;, &lt;table&gt;,
            &lt;column&gt;) and the function returns the integer SRID of the
            specified column by searching through the GEOMETRY_COLUMNS table.
            If the geometry column has not been properly added with the
            AddGeometryColumns() function, this function will not work
            either.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>force_collection(geometry)</term>

          <listitem>
            <para>Converts the geometry into a GEOMETRYCOLLECTION. This is
            useful for simplifying the WKB representation.</para>
          </listitem>
        </varlistentry>

        <varlistentry id="force_2d">
          <term>force_2d(geometry)</term>

          <listitem>
            <para>Forces the geometries into a "2-dimensional mode" so that
            all output representations will only have the X and Y coordinates.
            This is useful for force OGC-compliant output (since OGC only
            specifies 2-D geometries).</para>
          </listitem>
        </varlistentry>

        <varlistentry id="force_3d">
          <term>force_3d(geometry)</term>

          <listitem>
            <para>Forces the geometries into a "3-dimensional mode" so that
            all output representations will have the X, Y and Z
            coordinates.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>length2d(geometry)</term>

          <listitem>
            <para>Returns the 2-dimensional length of the geometry if it is a
            linestring or multi-linestring.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>length(geometry)</term>

          <listitem>
            <para>The length of this Curve in its associated spatial
            reference.</para>

            <para>synonym for length2d()</para>

            <para>OGC SPEC 2.1.5.1</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>length3d(geometry)</term>

          <listitem>
            <para>Returns the 3-dimensional length of the geometry if it is a
            linestring or multi-linestring.</para>
          </listitem>
        </varlistentry>

        <varlistentry id="length_spheroid">
          <term>length_spheroid(geometry,spheroid)</term>

          <listitem>
            <para>Calculates the length of of a geometry on an elipsoid. This
            is useful if the coordinates of the geometry are in
            latitude/longitude and a length is desired without reprojection.
            The elipsoid is a separate database type and can be constructed as
            follows:</para>

            <literallayout>SPHEROID[&lt;NAME&gt;,&lt;SEMI-MAJOR AXIS&gt;,&lt;INVERSE FLATTENING&gt;]</literallayout>

            <para>Eg:</para>

            <literallayout>SPHEROID["GRS_1980",6378137,298.257222101]</literallayout>

            <para>An example calculation might look like this:</para>

            <literallayout>SELECT
 length_spheroid(
  geometry_column,
  'SPHEROID["GRS_1980",6378137,298.257222101]'
 )
FROM geometry_table;</literallayout>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>length3d_spheroid(geometry,spheroid)</term>

          <listitem>
            <para>Calculates the length of of a geometry on an elipsoid,
            taking the elevation into account. This is just like
            length_spheroid except vertical coordinates (expressed in the same
            units as the spheroid axes) are used to calculate the extra
            distance vertical displacement adds.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>max_distance(linestring,linestring)</term>

          <listitem>
            <para>Returns the largest distance between two line
            strings.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>mem_size(geometry)</term>

          <listitem>
            <para>Returns the amount of space (in bytes) the geometry
            takes.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>multi(geometry)</term>

          <listitem>
            <para>Returns the geometry as a MULTI* geometry. If the geometry
            is already a MULTI*, it is returned unchanged.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>npoints(geometry)</term>

          <listitem>
            <para>Returns the number of points in the geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>nrings(geometry)</term>

          <listitem>
            <para>If the geometry is a polygon or multi-polygon returns the
            number of rings.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>numb_sub_objects(geometry)</term>

          <listitem>
            <para>Returns the number of objects stored in the geometry. This
            is useful for MULTI-geometries and GEOMETRYCOLLECTIONs.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>perimeter2d(geometry)</term>

          <listitem>
            <para>Returns the 2-dimensional perimeter of the geometry, if it
            is a polygon or multi-polygon.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>perimeter3d(geometry)</term>

          <listitem>
            <para>Returns the 3-dimensional perimeter of the geometry, if it
            is a polygon or multi-polygon.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>point_inside_circle(geometry,float,float,float)</term>

          <listitem>
            <para>The syntax for this functions is
            point_inside_circle(&lt;geometry&gt;,&lt;circle_center_x&gt;,&lt;circle_center_y&gt;,&lt;radius&gt;).
            Returns the true if the geometry is a point and is inside the
            circle. Returns false otherwise.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_version()</term>

          <listitem>
            <para>Returns the version number of the PostGIS functions
            installed in this database (deprecated, use postgis_full_version()
            instead).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_lib_version()</term>

          <listitem>
            <para>Returns the version number of the PostGIS library.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_scripts_installed()</term>

          <listitem>
            <para>Returns the version number of the postgis.sql script
            installed in this database.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_scripts_released()</term>

          <listitem>
            <para>Returns the version number of the postgis.sql script
            released with the installed postgis lib.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_geos_version()</term>

          <listitem>
            <para>Returns the version number of the GEOS library, or NULL if
            GEOS support is not enabled.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_proj_version()</term>

          <listitem>
            <para>Returns the version number of the PROJ4 library, or NULL if
            PROJ4 support is not enabled.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_uses_stats()</term>

          <listitem>
            <para>Returns true if STATS usage has been enabled, false
            otherwise.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>postgis_full_version()</term>

          <listitem>
            <para>Reports full postgis version and build configuration
            infos.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>summary(geometry)</term>

          <listitem>
            <para>Returns a text summary of the contents of the
            geometry.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>transform(geometry,integer)</term>

          <listitem>
            <para>Returns a new geometry with its coordinates transformed to
            the SRID referenced by the integer parameter. The destination SRID
            must exist in the SPATIAL_REF_SYS table.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>translate(geometry,float8,float8,float8)</term>

          <listitem>
            <para>Translates the geometry to a new location using the numeric
            parameters as offsets. Ie: translate(geom,X,Y,Z).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>xmin(box3d) ymin(box3d) zmin(box3d)</term>

          <listitem>
            <para>Returns the requested minima of a bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>xmax(box3d) ymax(box3d) zmax(box3d)</term>

          <listitem>
            <para>Returns the requested maxima of a bounding box.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>simplify(geometry, tolerance)</term>

          <listitem>
            <para>Returns a "simplified" version of the given geometry using
            the Douglas-Peuker algorithm. Will actually do something only with
            (multi)lines and (multi)polygons but you can safely call it with
            any kind of geometry. Since simplification occurs on a
            object-by-object basis you can also feed a GeometryCollection to
            this function. Note that returned geometry might loose its
            simplicity (see isSimple)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>line_interpolate_point(geometry, distance)</term>

          <listitem>
            <para>Interpolates a point along a line. First argument must be a
            LINESTRING. Second argument is a float between 0 and 1. Returns a
            point.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect1>
  </chapter>
</book>