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2215 lines
84 KiB
XML
2215 lines
84 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<chapter>
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<title>Using PostGIS</title>
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<sect1 id="RefObject">
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<title>GIS Objects</title>
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<para>The GIS objects supported by PostGIS are a superset of the "Simple
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Features" defined by the OpenGIS Consortium (OGC). As of version 0.9,
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PostGIS supports all the objects and functions specified in the OGC
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"Simple Features for SQL" specification.</para>
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<para>PostGIS extends the standard with support for 3DZ,3DM and 4D
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coordinates.</para>
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<sect2>
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<title>OpenGIS WKB and WKT</title>
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<para>The OpenGIS specification defines two standard ways of expressing
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spatial objects: the Well-Known Text (WKT) form and the Well-Known
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Binary (WKB) form. Both WKT and WKB include information about the type
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of the object and the coordinates which form the object.</para>
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<para>Examples of the text representations (WKT) of the spatial objects
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of the features are as follows:</para>
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<itemizedlist>
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<listitem>
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<para>POINT(0 0)</para>
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</listitem>
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<listitem>
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<para>LINESTRING(0 0,1 1,1 2)</para>
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</listitem>
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<listitem>
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<para>POLYGON((0 0,4 0,4 4,0 4,0 0),(1 1, 2 1, 2 2, 1 2,1 1))</para>
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</listitem>
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<listitem>
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<para>MULTIPOINT(0 0,1 2)</para>
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</listitem>
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<listitem>
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<para>MULTILINESTRING((0 0,1 1,1 2),(2 3,3 2,5 4))</para>
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</listitem>
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<listitem>
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<para>MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1)),
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((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))</para>
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</listitem>
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<listitem>
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<para>GEOMETRYCOLLECTION(POINT(2 3),LINESTRING(2 3,3 4))</para>
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</listitem>
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</itemizedlist>
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<para>The OpenGIS specification also requires that the internal storage
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format of spatial objects include a spatial referencing system
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identifier (SRID). The SRID is required when creating spatial objects
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for insertion into the database.</para>
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<para>Input/Output of these formats are available using the following
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interfaces:</para>
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<programlisting>bytea WKB = ST_AsBinary(geometry);
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text WKT = ST_AsText(geometry);
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geometry = ST_GeomFromWKB(bytea WKB, SRID);
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geometry = ST_GeometryFromText(text WKT, SRID);</programlisting>
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<para>For example, a valid insert statement to create and insert an OGC
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spatial object would be:</para>
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<programlisting>INSERT INTO geotable ( the_geom, the_name )
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VALUES ( ST_GeomFromText('POINT(-126.4 45.32)', 312), 'A Place');</programlisting>
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</sect2>
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<sect2>
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<title>PostGIS EWKB, EWKT and Canonical Forms</title>
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<para>OGC formats only support 2d geometries, and the associated SRID is
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*never* embedded in the input/output representations.</para>
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<para>PostGIS extended formats are currently superset of OGC one (every
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valid WKB/WKT is a valid EWKB/EWKT) but this might vary in the future,
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specifically if OGC comes out with a new format conflicting with our
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extensions. Thus you SHOULD NOT rely on this feature!</para>
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<para>PostGIS EWKB/EWKT add 3dm,3dz,4d coordinates support and embedded
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SRID information.</para>
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<para>Examples of the text representations (EWKT) of the extended
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spatial objects of the features are as follows:</para>
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<itemizedlist>
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<listitem>
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<para>POINT(0 0 0) -- XYZ</para>
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</listitem>
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<listitem>
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<para>SRID=32632;POINT(0 0) -- XY with SRID</para>
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</listitem>
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<listitem>
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<para>POINTM(0 0 0) -- XYM</para>
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</listitem>
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<listitem>
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<para>POINT(0 0 0 0) -- XYZM</para>
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</listitem>
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<listitem>
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<para>SRID=4326;MULTIPOINTM(0 0 0,1 2 1) -- XYM with SRID</para>
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</listitem>
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<listitem>
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<para>MULTILINESTRING((0 0 0,1 1 0,1 2 1),(2 3 1,3 2 1,5 4
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1))</para>
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</listitem>
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<listitem>
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<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
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0,1 1 0))</para>
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</listitem>
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<listitem>
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<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
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0,1 2 0,1 1 0)),((-1 -1 0,-1 -2 0,-2 -2 0,-2 -1 0,-1 -1 0)))</para>
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</listitem>
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<listitem>
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<para>GEOMETRYCOLLECTIONM(POINTM(2 3 9), LINESTRINGM(2 3 4, 3 4
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5))</para>
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</listitem>
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</itemizedlist>
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<para>Input/Output of these formats are available using the following
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interfaces:</para>
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<programlisting>bytea EWKB = ST_AsEWKB(geometry);
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text EWKT = ST_AsEWKT(geometry);
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geometry = ST_GeomFromEWKB(bytea EWKB);
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geometry = ST_GeomFromEWKT(text EWKT);</programlisting>
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<para>For example, a valid insert statement to create and insert a
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PostGIS spatial object would be:</para>
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<programlisting>INSERT INTO geotable ( the_geom, the_name )
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VALUES ( ST_GeomFromEWKT('SRID=312;POINTM(-126.4 45.32 15)'), 'A Place' )</programlisting>
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<para>The "canonical forms" of a PostgreSQL type are the representations
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you get with a simple query (without any function call) and the one
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which is guaranteed to be accepted with a simple insert, update or copy.
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For the postgis 'geometry' type these are: <programlisting>- Output
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- binary: EWKB
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ascii: HEXEWKB (EWKB in hex form)
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- Input
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- binary: EWKB
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ascii: HEXEWKB|EWKT </programlisting></para>
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<para>For example this statement reads EWKT and returns HEXEWKB in the
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process of canonical ascii input/output:</para>
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<programlisting>=# SELECT 'SRID=4;POINT(0 0)'::geometry;
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geometry
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----------------------------------------------------
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01010000200400000000000000000000000000000000000000
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(1 row)</programlisting>
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</sect2>
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<sect2>
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<title>SQL-MM Part 3</title>
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<para>The SQL Multimedia Applications Spatial specification extends the
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simple features for SQL spec by defining a number of circularly
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interpolated curves.</para>
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<para>The SQL-MM definitions include 3dm, 3dz and 4d coordinates, but do
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not allow the embedding of SRID information.</para>
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<para>The well-known text extensions are not yet fully supported.
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Examples of some simple curved geometries are shown below:</para>
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<itemizedlist>
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<listitem>
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<para>CIRCULARSTRING(0 0, 1 1, 1 0)</para>
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<para>CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0)</para>
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<para>The CIRCULARSTRING is the basic curve type, similar to a
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LINESTRING in the linear world. A single segment required three
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points, the start and end points (first and third) and any other
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point on the arc. The exception to this is for a closed circle,
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where the start and end points are the same. In this case the
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second point MUST be the center of the arc, ie the opposite side of
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the circle. To chain arcs together, the last point of the previous
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arc becomes the first point of the next arc, just like in
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LINESTRING. This means that a valid circular string must have an
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odd number of points greated than 1.</para>
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</listitem>
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<listitem>
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<para>COMPOUNDCURVE(CIRCULARSTRING(0 0, 1 1, 1 0),(1 0, 0 1))</para>
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<para>A compound curve is a single, continuous curve that has both
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curved (circular) segments and linear segments. That means that
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in addition to having well-formed components, the end point of
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every component (except the last) must be coincident with the
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start point of the following component.</para>
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</listitem>
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<listitem>
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<para>CURVEPOLYGON(CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0),(1 1, 3
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3, 3 1, 1 1))</para>
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<para>A CURVEPOLYGON is just like a polygon, with an outer ring
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and zero or more inner rings. The difference is that a ring can
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take the form of a circular string, linear string or compound
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string.</para>
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<para>This is currently where PostGIS falls down. Due to the way
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compound strings are represented internally, we cannot yet embed
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them within curve polygons.</para>
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</listitem>
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<listitem>
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<para>MULTICURVE((0 0, 5 5),CIRCULARSTRING(4 0, 4 4, 8 4))</para>
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<para>The MULTICURVE is a collection of curves, which can include
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linear strings, circular strings or compound strings.</para>
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</listitem>
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<listitem>
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<para>MULTISURFACE(CURVEPOLYGON(CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0
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0),(1 1, 3 3, 3 1, 1 1)),((10 10, 14 12, 11 10, 10 10),(11 11, 11.5
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11, 11 11.5, 11 11)))</para>
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<para>This is a collection of surfaces, which can be (linear)
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polygons or curve polygons.</para>
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</listitem>
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</itemizedlist>
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<note>
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<para>Currently, PostGIS cannot support the use of Compound Curves in
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a Curve Polygon.</para>
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</note>
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<note>
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<para>All floating point comparisons within the SQL-MM implementation
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are performed to a specified tolerance, currently 1E-8.</para>
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</note>
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</sect2>
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</sect1>
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<sect1>
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<title>Using OpenGIS Standards</title>
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<para>The OpenGIS "Simple Features Specification for SQL" defines standard
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GIS object types, the functions required to manipulate them, and a set of
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meta-data tables. In order to ensure that meta-data remain consistent,
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operations such as creating and removing a spatial column are carried out
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through special procedures defined by OpenGIS.</para>
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<para>There are two OpenGIS meta-data tables:
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<varname>SPATIAL_REF_SYS</varname> and
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<varname>GEOMETRY_COLUMNS</varname>. The
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<varname>SPATIAL_REF_SYS</varname> table holds the numeric IDs and textual
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descriptions of coordinate systems used in the spatial database.</para>
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<sect2 id="spatial_ref_sys">
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<title>The SPATIAL_REF_SYS Table and Spatial Reference Systems</title>
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<para>The spatial_ref_sys table is a PostGIS included and OGC compliant database table that lists over 3000
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known <ulink url="http://www.sharpgis.net/post/2007/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing.aspx">spatial reference systems</ulink>
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and details needed to transform/reproject between them.</para>
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<para>Although the PostGIS spatial_ref_sys table contains over 3000 of the more commonly used spatial reference system definitions that can be handled by the proj library, it does not contain all known to man and
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you can even define your own custom projection if you are familiar with proj4 constructs. Keep in mind that most spatial reference systems are regional and have no meaning when used outside of the bounds they were intended for.</para>
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<para>An excellent resource for finding spatial reference systems not defined in the core set is <ulink url="http://spatialreference.org/">http://spatialreference.org/</ulink></para>
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<para>Some of the more commonly used spatial reference systems are: <ulink url="http://spatialreference.org/ref/epsg/4326/">4326 - WGS 84 Long Lat</ulink>,
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<ulink url="http://spatialreference.org/ref/epsg/4269/">4269 - NAD 83 Long Lat</ulink>,
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<ulink url="http://spatialreference.org/ref/epsg/3395/">3395 - WGS 84 World Mercator</ulink>,
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<ulink url="http://spatialreference.org/ref/epsg/2163/">2163 - US National Atlas Equal Area</ulink>,
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Spatial reference systems for each NAD 83, WGS 84 UTM zone - UTM zones are one of the most ideal for measurement, but only cover 6-degree regions.
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</para>
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<para>
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Various US state plane spatial reference systems (meter or feet based) - usually one or 2 exists per US state. Most of the meter ones are in the core set, but many of the
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feet based ones or ESRI created ones you will need to pull from <ulink url="http://spatialreference.org">spatialreference.org</ulink>.
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</para>
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<para>
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For details on determining which UTM zone to use for your area of interest, check out the <ulink url="/support/wiki/index.php?plpgsqlfunctions">utmzone PostGIS plpgsql helper function</ulink>.
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</para>
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<para>The <varname>SPATIAL_REF_SYS</varname> table definition is as
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follows:</para>
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<programlisting>CREATE TABLE spatial_ref_sys (
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srid INTEGER NOT NULL PRIMARY KEY,
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auth_name VARCHAR(256),
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auth_srid INTEGER,
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srtext VARCHAR(2048),
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proj4text VARCHAR(2048)
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)</programlisting>
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<para>The <varname>SPATIAL_REF_SYS</varname> columns are as
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follows:</para>
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<variablelist>
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<varlistentry>
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<term><ulink url="http://en.wikipedia.org/wiki/SRID">SRID</ulink></term>
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<listitem>
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<para>An integer value that uniquely identifies the Spatial
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Referencing System (SRS) within the database.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>AUTH_NAME</term>
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<listitem>
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<para>The name of the standard or standards body that is being
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cited for this reference system. For example, "EPSG" would be a
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valid <varname>AUTH_NAME</varname>.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>AUTH_SRID</term>
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<listitem>
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<para>The ID of the Spatial Reference System as defined by the
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Authority cited in the <varname>AUTH_NAME</varname>. In the case
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of EPSG, this is where the EPSG projection code would go.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>SRTEXT</term>
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<listitem>
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<para>The Well-Known Text representation of the Spatial Reference
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System. An example of a WKT SRS representation is:</para>
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<programlisting>PROJCS["NAD83 / UTM Zone 10N",
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GEOGCS["NAD83",
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DATUM["North_American_Datum_1983",
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SPHEROID["GRS 1980",6378137,298.257222101]
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],
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PRIMEM["Greenwich",0],
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UNIT["degree",0.0174532925199433]
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],
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PROJECTION["Transverse_Mercator"],
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PARAMETER["latitude_of_origin",0],
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PARAMETER["central_meridian",-123],
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PARAMETER["scale_factor",0.9996],
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PARAMETER["false_easting",500000],
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PARAMETER["false_northing",0],
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UNIT["metre",1]
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]</programlisting>
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<para>For a listing of EPSG projection codes and their
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corresponding WKT representations, see <ulink
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url="http://www.opengeospatial.org/">http://www.opengeospatial.org/</ulink>.
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For a discussion of WKT in general, see the OpenGIS "Coordinate
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Transformation Services Implementation Specification" at <ulink
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url="http://www.opengeospatial.org/standards">http://www.opengeospatial.org/standards</ulink>.
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For information on the European Petroleum Survey Group (EPSG) and
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their database of spatial reference systems, see <ulink
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url="http://www.epsg.org/">http://www.epsg.org</ulink>.</para>
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||
</listitem>
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||
</varlistentry>
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||
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<varlistentry>
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<term>PROJ4TEXT</term>
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<listitem>
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<para>PostGIS uses the Proj4 library to provide coordinate
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transformation capabilities. The <varname>PROJ4TEXT</varname>
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column contains the Proj4 coordinate definition string for a
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particular SRID. For example:</para>
|
||
|
||
<programlisting>+proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m</programlisting>
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||
|
||
<para>For more information about, see the Proj4 web site at <ulink
|
||
url="http://trac.osgeo.org/proj/">http://trac.osgeo.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 VARRCHAR(256) NOT NULL,
|
||
f_table_schema VARCHAR(256) NOT NULL,
|
||
f_table_nam 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 PostgreSQL schema name is
|
||
used (<varname>public</varname> is the default).</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, 3 or 4 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 <varname>SPATIAL_REF_SYS</varname>.</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,
|
||
MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION or
|
||
corresponding XYM versions POINTM, LINESTRINGM, POLYGONM,
|
||
MULTIPOINTM, MULTILINESTRINGM, MULTIPOLYGONM, GEOMETRYCOLLECTIONM.
|
||
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 id="Create_Spatial_Table">
|
||
<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: <command>CREATE TABLE ROADS_GEOM ( ID int4, NAME
|
||
varchar(25) )</command></para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>Add a spatial column to the table using the OpenGIS
|
||
"AddGeometryColumn" function.</para>
|
||
|
||
<para>The syntax is: <programlisting>AddGeometryColumn(
|
||
<schema_name>,
|
||
<table_name>,
|
||
<column_name>,
|
||
<srid>,
|
||
<type>,
|
||
<dimension>
|
||
)</programlisting> Or, using current schema: <programlisting>AddGeometryColumn(
|
||
<table_name>,
|
||
<column_name>,
|
||
<srid>,
|
||
<type>,
|
||
<dimension>
|
||
)</programlisting></para>
|
||
|
||
<para>Example1: <command>SELECT AddGeometryColumn('public',
|
||
'roads_geom', 'geom', 423, 'LINESTRING', 2)</command></para>
|
||
|
||
<para>Example2: <command>SELECT AddGeometryColumn( 'roads_geom',
|
||
'geom', 423, 'LINESTRING', 2)</command></para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>Here is an example of SQL used to create a table and add a spatial
|
||
column (assuming that an SRID of 128 exists already):</para>
|
||
|
||
<programlisting>CREATE TABLE parks (
|
||
park_id INTEGER,
|
||
park_name VARCHAR,
|
||
park_date DATE,
|
||
park_type VARCHAR
|
||
);
|
||
SELECT AddGeometryColumn('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 INTEGER,
|
||
road_name VARCHAR
|
||
);
|
||
SELECT AddGeometryColumn( 'roads', 'roads_geom', -1, 'GEOMETRY', 3 );</programlisting>
|
||
</sect2>
|
||
|
||
<sect2 id="Manual_Register_Spatial_Column">
|
||
<title>Manually Registering Geometry Columns in geometry_columns</title>
|
||
<para>The AddGeometryColumn() approach creates a geometry column and also registers the new
|
||
column in the geometry_columns table. If your software utilizes geometry_columns, then
|
||
any geometry columns you need to query by must be registered in this table. Two of the cases
|
||
where you want a geometry column to be registered in the geometry_columns table, but you can't use
|
||
AddGeometryColumn, is in the case of SQL Views and bulk inserts. For these cases, you must register the column in the
|
||
geometry_columns table manually. Below is a simple script to do that.</para>
|
||
|
||
<programlisting>
|
||
--Lets say you have a view created like this
|
||
CREATE VIEW public.vwmytablemercator AS
|
||
SELECT gid, ST_Transform(the_geom,3395) As the_geom, f_name
|
||
FROM public.mytable;
|
||
|
||
--To register this table in AddGeometry columns - do the following
|
||
INSERT INTO geometry_columns(f_table_catalog, f_table_schema, f_table_name, f_geometry_column, coord_dimension, srid, "type")
|
||
SELECT '', 'public', 'vwmytablemercator', 'the_geom', ST_CoordDim(the_geom), ST_SRID(the_geom), GeometryType(the_geom)
|
||
FROM public.vwmytablemercator LIMIT 1;
|
||
|
||
</programlisting>
|
||
|
||
<programlisting>
|
||
--Lets say you created a derivative table by doing a bulk insert
|
||
SELECT poi.gid, poi.the_geom, citybounds.city_name
|
||
INTO myschema.myspecialpois
|
||
FROM poi INNER JOIN citybounds ON ST_Intersects(citybounds.the_geom, poi.the_geom);
|
||
|
||
--Create index on new table
|
||
CREATE INDEX idx_myschema_myspecialpois_geom_gist
|
||
ON myschema.myspecialpois USING gist(the_geom);
|
||
|
||
--To manually register this new table's geometry column in geometry_columns
|
||
-- we do the same thing as with view
|
||
INSERT INTO geometry_columns(f_table_catalog, f_table_schema, f_table_name, f_geometry_column, coord_dimension, srid, "type")
|
||
SELECT '', 'myschema', 'myspecialpois', 'the_geom', ST_CoordDim(the_geom), ST_SRID(the_geom), GeometryType(the_geom)
|
||
FROM public.myschema.myspecialpois LIMIT 1;
|
||
|
||
</programlisting>
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="OGC_Validity">
|
||
<title>Ensuring OpenGIS compliancy of geometries</title>
|
||
|
||
<para>PostGIS is compliant with the Open Geospatial Consortium’s (OGC)
|
||
OpenGIS Specifications. As such, many PostGIS methods require, or more
|
||
accurately, assume that geometries that are operated on are both simple
|
||
and valid. for example, it does not make sense to calculate the area of
|
||
a polygon that has a hole defined outside of the polygon, or to construct
|
||
a polygon from a non-simple boundary line.</para>
|
||
|
||
<para>According to the OGC Specifications, a <emphasis>simple</emphasis>
|
||
geometry is one that has no anomalous geometric points, such as self
|
||
intersection or self tangency and primarily refers to 0 or 1-dimensional
|
||
geometries (i.e. <varname>[MULTI]POINT, [MULTI]LINESTRING</varname>).
|
||
Geometry validity, on the other hand, primarily refers to 2-dimensional
|
||
geometries (i.e. <varname>[MULTI]POLYGON)</varname> and defines the set
|
||
of assertions that characterizes a valid polygon. The description of each
|
||
geometric class includes specific conditions that further detail geometric
|
||
simplicity and validity.</para>
|
||
|
||
<para>A <varname>POINT</varname> is inheritably <emphasis>simple</emphasis>
|
||
as a 0-dimensional geometry object.</para>
|
||
|
||
<para><varname>MULTIPOINT</varname>s are <emphasis>simple</emphasis> if
|
||
no two coordinates (<varname>POINT</varname>s) are equal (have identical
|
||
coordinate values).</para>
|
||
|
||
<para>A <varname>LINESTRING</varname> is <emphasis>simple</emphasis> if
|
||
it does not pass through the same <varname>POINT</varname> twice (except
|
||
for the endpoints, in which case it is referred to as a linear ring and
|
||
additionally considered closed).</para>
|
||
|
||
<informaltable>
|
||
<tgroup cols="2" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple01.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(a)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple02.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(b)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple03.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(c)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple04.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(d)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis role="bold">(a)</emphasis> and
|
||
<emphasis role="bold">(c)</emphasis> are simple
|
||
<varname>LINESTRING</varname>s, <emphasis role="bold">(b)</emphasis>
|
||
and <emphasis role="bold">(d)</emphasis> are not.</para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>A <varname>MULTILINESTRING</varname> is <emphasis>simple</emphasis>
|
||
only if all of its elements are simple and the only intersection between
|
||
any two elements occurs at <varname>POINT</varname>s that are on the
|
||
boundaries of both elements. </para>
|
||
|
||
<informaltable>
|
||
<tgroup cols="3" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple05.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(e)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple06.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(f)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple07.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(g)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis role="bold">(e)</emphasis> and
|
||
<emphasis role="bold">(f)</emphasis> are simple
|
||
<varname>MULTILINESTRING</varname>s, <emphasis role="bold">(g)</emphasis>
|
||
is not.</para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>By definition, a <varname>POLYGON</varname> is always
|
||
<emphasis>simple</emphasis>. It is <emphasis>valid</emphasis> if no two
|
||
rings in the boundary (made up of an exterior ring and interior rings)
|
||
cross. The boundary of a <varname>POLYGON</varname> may intersect at a
|
||
<varname>POINT</varname> but only as a tangent (i.e. not on a line).
|
||
A <varname>POLYGON</varname> may not have cut lines or spikes and the
|
||
interior rings must be contained entirely within the exterior ring.</para>
|
||
|
||
<informaltable>
|
||
<tgroup cols="3" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid01.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(h)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid02.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(i)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid03.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(j)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
<row>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid04.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(k)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid05.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(l)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid06.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(m)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis role="bold">(h)</emphasis> and
|
||
<emphasis role="bold">(i)</emphasis> are valid
|
||
<varname>POLYGON</varname>s, <emphasis role="bold">(j-m)</emphasis>
|
||
cannot be represented as single <varname>POLYGON</varname>s, but
|
||
<emphasis role="bold">(j)</emphasis> and <emphasis role="bold">(m)</emphasis>
|
||
could be represented as a valid <varname>MULTIPOLYGON</varname>.
|
||
</para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>A <varname>MULTIPOLYGON</varname> is <emphasis>valid</emphasis>
|
||
if and only if all of its elements are valid and the interiors of no two
|
||
elements intersect. The boundaries of any two elements may touch, but
|
||
only at a finite number of <varname>POINT</varname>s.</para>
|
||
|
||
<informaltable>
|
||
<tgroup cols="2" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid07.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(n)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid08.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(o)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis role="bold">(n)</emphasis> and
|
||
<emphasis role="bold">(o)</emphasis> are not valid
|
||
<varname>MULTIPOLYGON</varname>s.</para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>Most of the functions implemented by the GEOS library rely on the
|
||
assumption that your geometries are valid as specified by the OpenGIS
|
||
Simple Feature Specification. To check simplicity or validity of
|
||
geometries you can use the <link linkend="ST_IsSimple">ST_IsSimple()</link> and
|
||
<link linkend="ST_IsValid">ST_IsValid()</link></para>
|
||
|
||
<programlisting>-- Typically, it doesn't make sense to check
|
||
-- for validity on linear features since it will always return TRUE.
|
||
-- But in this example, PostGIS extends the definition of the OGC IsValid
|
||
-- by returning false if a LinearRing (start and end points are the same)
|
||
-- has less than 2 vertices.
|
||
gisdb=# SELECT
|
||
st_isvalid('LINESTRING(0 0, 1 1)'),
|
||
st_isvalid('LINESTRING(0 0, 0 0)');
|
||
|
||
st_isvalid | st_isvalid
|
||
------------+---------
|
||
t | f</programlisting>
|
||
|
||
<para>By default, PostGIS does not apply this validity check on geometry
|
||
input, because testing for validity needs lots of CPU time for complex
|
||
geometries, especially polygons. If you do not trust your data sources,
|
||
you can manually enforce such a check to your tables by adding a check
|
||
constraint:</para>
|
||
|
||
<programlisting>ALTER TABLE mytable
|
||
ADD CONSTRAINT geometry_valid_check
|
||
CHECK (isvalid(the_geom));</programlisting>
|
||
|
||
<para>If you encounter any strange error messages such as "GEOS
|
||
Intersection() threw an error!" or "JTS Intersection() threw an error!"
|
||
when calling PostGIS functions with valid input geometries, you likely
|
||
found an error in either PostGIS or one of the libraries it uses, and
|
||
you should contact the PostGIS developers. The same is true if a PostGIS
|
||
function returns an invalid geometry for valid input.</para>
|
||
|
||
<note>
|
||
<para>Strictly compliant OGC geometries cannot have Z or M values. The
|
||
<link linkend="ST_IsValid">ST_IsValid()</link> function won't consider
|
||
higher dimensioned geometries invalid! Invocations of <link
|
||
linkend="AddGeometryColumn">AddGeometryColumn()</link> will add a
|
||
constraint checking geometry dimensions, so it is enough to specify 2
|
||
there.</para>
|
||
</note>
|
||
</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 (road_id, roads_geom, road_name)
|
||
VALUES (1,ST_GeomFromText('LINESTRING(191232 243118,191108 243242)',-1),'Jeff Rd');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (2,ST_GeomFromText('LINESTRING(189141 244158,189265 244817)',-1),'Geordie Rd');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (3,ST_GeomFromText('LINESTRING(192783 228138,192612 229814)',-1),'Paul St');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (4,ST_GeomFromText('LINESTRING(189412 252431,189631 259122)',-1),'Graeme Ave');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (5,ST_GeomFromText('LINESTRING(190131 224148,190871 228134)',-1),'Phil Tce');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (6,ST_GeomFromText('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>-p</term>
|
||
|
||
<listitem>
|
||
<para>Only produces the table creation SQL code, without adding
|
||
any actual data. This can be used if you need to completely
|
||
separate the table creation and data loading steps.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-D</term>
|
||
|
||
<listitem>
|
||
<para>Use the PostgreSQL "dump" format for the output data. This
|
||
can be combined with -a, -c and -d. It is much faster to load than
|
||
the default "insert" SQL format. Use this for very large data
|
||
sets.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-s <SRID></term>
|
||
|
||
<listitem>
|
||
<para>Creates and populates the geometry tables with the specified
|
||
SRID.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-k</term>
|
||
|
||
<listitem>
|
||
<para>Keep identifiers' case (column, schema and attributes). Note
|
||
that attributes in Shapefile are all UPPERCASE.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-i</term>
|
||
|
||
<listitem>
|
||
<para>Coerce all integers to standard 32-bit integers, do not
|
||
create 64-bit bigints, even if the DBF header signature appears to
|
||
warrant it.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-I</term>
|
||
|
||
<listitem>
|
||
<para>Create a GiST index on the geometry column.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-w</term>
|
||
|
||
<listitem>
|
||
<para>Output WKT format, for use with older (0.x) versions of
|
||
PostGIS. Note that this will introduce coordinate drifts and will
|
||
drop M values from shapefiles.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-W <encoding></term>
|
||
|
||
<listitem>
|
||
<para>Specify encoding of the input data (dbf file). When used,
|
||
all attributes of the dbf are converted from the specified
|
||
encoding to UTF8. The resulting SQL output will contain a
|
||
<code>SET CLIENT_ENCODING to UTF8</code> command, so that the
|
||
backend will be able to reconvert from UTF8 to whatever encoding
|
||
the database is configured to use internally.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
|
||
<para>Note that -a, -c, -d and -p are mutually exclusive.</para>
|
||
|
||
<para>An example session using the loader to create an input file and
|
||
uploading it might look like this:</para>
|
||
|
||
<programlisting># shp2pgsql shaperoads myschema.roadstable > 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 myschema.roadstable | 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 road_id, ST_AsText(road_geom) AS geom, road_name FROM roads;
|
||
|
||
road_id | geom | road_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>&&</term>
|
||
|
||
<listitem>
|
||
<para>This operator tells whether the bounding box of one geometry
|
||
intersects 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 two 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
|
||
"GeomFromText()" function. So, for example:</para>
|
||
|
||
<programlisting>SELECT road_id, road_name
|
||
FROM roads
|
||
WHERE roads_geom ~= ST_GeomFromText('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 "&&" 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 road_id, road_name
|
||
FROM roads
|
||
WHERE roads_geom && ST_GeomFromText('POLYGON((...))',-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 ST_AsText(roads_geom) AS geom
|
||
FROM roads
|
||
WHERE
|
||
roads_geom && SetSRID('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 (possibly defined by a query) into
|
||
a shape file. The basic syntax is:</para>
|
||
|
||
<programlisting>pgsql2shp [<options>] <database> [<schema>.]<table></programlisting>
|
||
|
||
<programlisting>pgsql2shp [<options>] <database> <query></programlisting>
|
||
|
||
<para>The commandline options are:</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>-f <filename></term>
|
||
|
||
<listitem>
|
||
<para>Write the output to a particular filename.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-h <host></term>
|
||
|
||
<listitem>
|
||
<para>The database host to connect to.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-p <port></term>
|
||
|
||
<listitem>
|
||
<para>The port to connect to on the database host.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-P <password></term>
|
||
|
||
<listitem>
|
||
<para>The password to use when connecting to the database.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-u <user></term>
|
||
|
||
<listitem>
|
||
<para>The username to use when connecting to the database.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-g <geometry column></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>
|
||
|
||
<varlistentry>
|
||
<term>-b</term>
|
||
|
||
<listitem>
|
||
<para>Use a binary cursor. This will make the operation faster,
|
||
but will not work if any NON-geometry attribute in the table lacks
|
||
a cast to text.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-r</term>
|
||
|
||
<listitem>
|
||
<para>Raw mode. Do not drop the <varname>gid</varname> field, or
|
||
escape column names.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-d</term>
|
||
|
||
<listitem>
|
||
<para>For backward compatibility: write a 3-dimensional shape file
|
||
when dumping from old (pre-1.0.0) postgis databases (the default
|
||
is to write a 2-dimensional shape file in that case). Starting
|
||
from postgis-1.0.0+, dimensions are fully encoded.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</sect2>
|
||
</sect1>
|
||
|
||
<sect1>
|
||
<title>Building Indexes</title>
|
||
|
||
<para>Indexes are what make using a spatial database for large data sets
|
||
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 generic 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] ); </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
|
||
<command>update_geometry_stats([table_name, column_name])</command>
|
||
(compute distribution) and <command>VACUUM ANALYZE [table_name]
|
||
[column_name]</command> (compute number of values). Starting with
|
||
PostgreSQL 8.0 running <command>VACUUM ANALYZE</command> will do
|
||
both operations. You should regularly vacuum your databases anyways
|
||
-- many PostgreSQL DBAs have <command>VACUUM</command> 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 <command>SET
|
||
ENABLE_SEQSCAN=OFF</command> 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 <varname>ENABLE_SEQSCAN</varname> 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
|
||
<varname>ENABLE_SEQSCAN</varname>.</para>
|
||
</note>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>If you find the planner wrong about the cost of sequential vs
|
||
index scans try reducing the value of random_page_cost in
|
||
postgresql.conf or using SET random_page_cost=#. Default value for
|
||
the parameter is 4, try setting it to 1 or 2. Decrementing the value
|
||
makes the planner more inclined of using Index scans.</para>
|
||
</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 && can take advantage
|
||
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 ST_Distance(the_geom, ST_GeomFromText('POINT(100000 200000)', -1)) < 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>ST_Distance()</varname> calculation
|
||
for each row in the table. We can avoid this by using the &&
|
||
operator to reduce the number of distance calculations required:</para>
|
||
|
||
<programlisting>SELECT the_geom
|
||
FROM geom_table
|
||
WHERE the_geom && 'BOX3D(90900 190900, 100100 200100)'::box3d
|
||
AND
|
||
ST_Distance(the_geom, ST_GeomFromText('POINT(100000 200000)', -1)) < 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 && operation is a 200 unit square box centered on
|
||
the original point - this is our "query box". The && 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>
|
||
|
||
<note>
|
||
<title>Change in Behavior</title>
|
||
|
||
<para>As of PostGIS 1.3.0, most of the Geometry Relationship
|
||
Functions, with the notable exceptions of ST_Disjoint and ST_Relate,
|
||
include implicit bounding box overlap operators.</para>
|
||
</note>
|
||
</sect2>
|
||
|
||
<sect2>
|
||
<title>Examples of Spatial SQL</title>
|
||
|
||
<para>The examples in this section will make use of two tables, a table
|
||
of linear roads, and a table of polygonal municipality boundaries. The
|
||
table definitions for the <varname>bc_roads</varname> table is:</para>
|
||
|
||
<programlisting>Column | Type | Description
|
||
------------+-------------------+-------------------
|
||
gid | integer | Unique ID
|
||
name | character varying | Road Name
|
||
the_geom | geometry | Location Geometry (Linestring)</programlisting>
|
||
|
||
<para>The table definition for the <varname>bc_municipality</varname>
|
||
table is:</para>
|
||
|
||
<programlisting>Column | Type | Description
|
||
-----------+-------------------+-------------------
|
||
gid | integer | Unique ID
|
||
code | integer | Unique ID
|
||
name | character varying | City / Town Name
|
||
the_geom | geometry | Location Geometry (Polygon)</programlisting>
|
||
|
||
<qandaset>
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the total length of all roads, expressed in
|
||
kilometers?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>You can answer this question with a very simple piece of
|
||
SQL:</para>
|
||
|
||
<programlisting>SELECT sum(ST_Length(the_geom))/1000 AS km_roads FROM bc_roads;
|
||
|
||
km_roads
|
||
------------------
|
||
70842.1243039643
|
||
(1 row)</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>How large is the city of Prince George, in hectares?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>This query combines an attribute condition (on the
|
||
municipality name) with a spatial calculation (of the
|
||
area):</para>
|
||
|
||
<programlisting>SELECT
|
||
ST_Area(the_geom)/10000 AS hectares
|
||
FROM bc_municipality
|
||
WHERE name = 'PRINCE GEORGE';
|
||
|
||
hectares
|
||
------------------
|
||
32657.9103824927
|
||
(1 row)</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the largest municipality in the province, by
|
||
area?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>This query brings a spatial measurement into the query
|
||
condition. There are several ways of approaching this problem, but
|
||
the most efficient is below:</para>
|
||
|
||
<programlisting>SELECT
|
||
name,
|
||
ST_Area(the_geom)/10000 AS hectares
|
||
FROM
|
||
bc_municipality
|
||
ORDER BY hectares DESC
|
||
LIMIT 1;
|
||
|
||
name | hectares
|
||
---------------+-----------------
|
||
TUMBLER RIDGE | 155020.02556131
|
||
(1 row)</programlisting>
|
||
|
||
<para>Note that in order to answer this query we have to calculate
|
||
the area of every polygon. If we were doing this a lot it would
|
||
make sense to add an area column to the table that we could
|
||
separately index for performance. By ordering the results in a
|
||
descending direction, and them using the PostgreSQL "LIMIT"
|
||
command we can easily pick off the largest value without using an
|
||
aggregate function like max().</para>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the length of roads fully contained within each
|
||
municipality?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>This is an example of a "spatial join", because we are
|
||
bringing together data from two tables (doing a join) but using a
|
||
spatial interaction condition ("contained") as the join condition
|
||
rather than the usual relational approach of joining on a common
|
||
key:</para>
|
||
|
||
<programlisting>SELECT
|
||
m.name,
|
||
sum(ST_Length(r.the_geom))/1000 as roads_km
|
||
FROM
|
||
bc_roads AS r,
|
||
bc_municipality AS m
|
||
WHERE
|
||
ST_Contains(m.the_geom,r.the_geom)
|
||
GROUP BY m.name
|
||
ORDER BY roads_km;
|
||
|
||
name | roads_km
|
||
----------------------------+------------------
|
||
SURREY | 1539.47553551242
|
||
VANCOUVER | 1450.33093486576
|
||
LANGLEY DISTRICT | 833.793392535662
|
||
BURNABY | 773.769091404338
|
||
PRINCE GEORGE | 694.37554369147
|
||
...</programlisting>
|
||
|
||
<para>This query takes a while, because every road in the table is
|
||
summarized into the final result (about 250K roads for our
|
||
particular example table). For smaller overlays (several thousand
|
||
records on several hundred) the response can be very fast.</para>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>Create a new table with all the roads within the city of
|
||
Prince George.</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>This is an example of an "overlay", which takes in two
|
||
tables and outputs a new table that consists of spatially clipped
|
||
or cut resultants. Unlike the "spatial join" demonstrated above,
|
||
this query actually creates new geometries. An overlay is like a
|
||
turbo-charged spatial join, and is useful for more exact analysis
|
||
work:</para>
|
||
|
||
<programlisting>CREATE TABLE pg_roads as
|
||
SELECT
|
||
ST_Intersection(r.the_geom, m.the_geom) AS intersection_geom,
|
||
ST_Length(r.the_geom) AS rd_orig_length,
|
||
r.*
|
||
FROM
|
||
bc_roads AS r,
|
||
bc_municipality AS m
|
||
WHERE m.name = 'PRINCE GEORGE' AND ST_Intersects(r.the_geom, m.the_geom);</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the length in kilometers of "Douglas St" in
|
||
Victoria?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<programlisting>SELECT
|
||
sum(ST_Length(r.the_geom))/1000 AS kilometers
|
||
FROM
|
||
bc_roads r,
|
||
bc_municipality m
|
||
WHERE r.name = 'Douglas St' AND m.name = 'VICTORIA'
|
||
AND ST_Contains(m.the_geom, r.the_geom) ;
|
||
|
||
kilometers
|
||
------------------
|
||
4.89151904172838
|
||
(1 row)</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the largest municipality polygon that has a
|
||
hole?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<programlisting>SELECT gid, name, ST_Area(the_geom) AS area
|
||
FROM bc_municipality
|
||
WHERE ST_NRings(the_geom) > 1
|
||
ORDER BY area DESC LIMIT 1;
|
||
|
||
gid | name | area
|
||
-----+--------------+------------------
|
||
12 | SPALLUMCHEEN | 257374619.430216
|
||
(1 row)</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
</qandaset>
|
||
</sect2>
|
||
</sect1>
|
||
|
||
<sect1 id="Using_Mapserver">
|
||
<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.opengeospatial.org/standards">http://www.opengeospatial.org/standards</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"
|
||
PROCESSING "CLOSE_CONNECTION=DEFER"
|
||
# 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 >= 4"
|
||
CLASS
|
||
# Make the superhighways brighter and 2 pixels wide
|
||
EXPRESSION ([numlanes] >= 6)
|
||
STYLE
|
||
COLOR 255 22 22
|
||
WIDTH 2
|
||
END
|
||
END
|
||
CLASS
|
||
# All the rest are darker and only 1 pixel wide
|
||
EXPRESSION ([numlanes] < 6)
|
||
STYLE
|
||
COLOR 205 92 82
|
||
END
|
||
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 <>):</para>
|
||
|
||
<para>user=<username> password=<password>
|
||
dbname=<username> hostname=<server>
|
||
port=<5432></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 "<column> from
|
||
<tablename>" where the column is the spatial column to
|
||
be rendered to the map.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>PROCESSING</term>
|
||
|
||
<listitem>
|
||
<para>Putting in a CLOSE_CONNECTION=DEFER if you have multiple layers reuses existing connections instead of closing them. This improves
|
||
speed. Refer to for <ulink url="http://blog.cleverelephant.ca/2008/10/mapserverpostgis-performance-tips.html">Paul's Mapserver PostGIS Performance Tips</ulink> for more detailed explanation. </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 >= 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] );</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>Frequently Asked Questions</title>
|
||
|
||
<qandaset>
|
||
<qandaentry>
|
||
<question>
|
||
<para>When I use an <varname>EXPRESSION</varname> in my map file,
|
||
the condition never returns as true, even though I know the values
|
||
exist in my table.</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>Unlike shape files, PostGIS field names have to be
|
||
referenced in EXPRESSIONS using <emphasis>lower
|
||
case</emphasis>.</para>
|
||
|
||
<programlisting>EXPRESSION ([numlanes] >= 6)</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>The FILTER I use for my Shape files is not working for my
|
||
PostGIS table of the same data.</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>Unlike shape files, filters for PostGIS layers use SQL
|
||
syntax (they are appended to the SQL statement the PostGIS
|
||
connector generates for drawing layers in Mapserver).</para>
|
||
|
||
<programlisting>FILTER "type = 'highway' and numlanes >= 4"</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>My PostGIS layer draws much slower than my Shape file layer,
|
||
is this normal?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>In general, expect PostGIS layers to be 10% slower than
|
||
equivalent Shape files layers, due to the extra overhead involved
|
||
in database connections, data transformations and data transit
|
||
between the database and Mapserver.</para>
|
||
|
||
<para>If you are finding substantial draw performance problems, it
|
||
is likely that you have not build a spatial index on your
|
||
table.</para>
|
||
|
||
<programlisting>postgis# CREATE INDEX geotable_gix ON geotable USING GIST ( geocolumn );
|
||
postgis# SELECT update_geometry_stats(); -- For PGSQL < 8.0
|
||
postgis# VACUUM ANALYZE; -- For PGSQL >= 8.0</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>My PostGIS layer draws fine, but queries are really slow.
|
||
What is wrong?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>For queries to be fast, you must have a unique key for your
|
||
spatial table and you must have an index on that unique
|
||
key.</para>
|
||
|
||
<para>You can specify what unique key for mapserver to use with
|
||
the <varname>USING UNIQUE</varname> clause in your
|
||
<varname>DATA</varname> line:</para>
|
||
|
||
<programlisting>DATA "the_geom FROM geotable USING UNIQUE gid"</programlisting>
|
||
|
||
<para>If your table does not have an explicit unique column, you
|
||
can "fake" a unique column by using the PostgreSQL row "oid" for
|
||
your unique column. "oid" is the default unique column if you do
|
||
not declare one, so enhancing your query speed is a matter of
|
||
building an index on your spatial table oid value.</para>
|
||
|
||
<programlisting>postgis# CREATE INDEX geotable_oid_idx ON geotable (oid);</programlisting>
|
||
</answer>
|
||
</qandaentry>
|
||
</qandaset>
|
||
</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 <uniqueid></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=<srid></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>
|
||
|
||
<warning>
|
||
<para>The parser for Mapserver PostGIS layers is fairly primitive, and
|
||
is case sensitive in a few areas. Be careful to ensure that all SQL
|
||
keywords and all your <varname>USING</varname> clauses are in upper
|
||
case, and that your <varname>USING UNIQUE</varname> clause precedes
|
||
your <varname>USING SRID</varname> clause.</para>
|
||
</warning>
|
||
</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
|
||
STYLE
|
||
COLOR 0 0 0
|
||
END
|
||
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 achieve this
|
||
effect:</para>
|
||
|
||
<programlisting>LAYER
|
||
CONNECTIONTYPE postgis
|
||
CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
|
||
PROCESSING "CLOSE_CONNECTION=DEFER"
|
||
DATA "the_geom FROM roads"
|
||
MINSCALE 100000
|
||
STATUS ON
|
||
TYPE LINE
|
||
FILTER "road_type = 'highway'"
|
||
CLASS
|
||
COLOR 0 0 0
|
||
END
|
||
END
|
||
LAYER
|
||
CONNECTIONTYPE postgis
|
||
CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
|
||
PROCESSING "CLOSE_CONNECTION=DEFER"
|
||
DATA "the_geom FROM roads"
|
||
MAXSCALE 100000
|
||
STATUS ON
|
||
TYPE LINE
|
||
CLASSITEM road_type
|
||
CLASS
|
||
EXPRESSION "highway"
|
||
STYLE
|
||
WIDTH 2
|
||
COLOR 255 0 0
|
||
END
|
||
END
|
||
CLASS
|
||
STYLE
|
||
COLOR 0 0 0
|
||
END
|
||
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
|
||
CONNECTIONTYPE postgis
|
||
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 ST_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 < pl.numRings(); r++) {
|
||
LinearRing rng = pl.getRing(r);
|
||
System.out.println("Ring: " + r);
|
||
for( int p = 0; p < 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>
|