postgis/lwgeom

Welcome to the Initial version of LWGEOM.

More information is available on the PostGIS user's mailing list and 
the PostGIS developer's mailing list.  

Differences
-----------

The LWGEOM is very much like the original PostGIS GEOMETRY type.  The 
main differences are:

a) LWGEOMs are much smaller than the PostGIS GEOMETRY
b) LWGEOMs natively support 2d, 3d, and 4d points
c) LWGEOMs are indexed using single-precision bounding boxes.  This
   make the indexes significantly smaller than PostGIS GEOMETRY
   indexes.
d) LWGEOMs are internally very similar to OGC WKB   
e) The "normal" view of LWGEOMs is OGC WKB - PostGIS GEOMETRY is OGC WKT
f) PostGIS geometries have a built-in BOX3D.  LWGEOMs have an *optional*
   BOX2D (see below).


Also included with the LWGEOMs is a type called 'box2d'.  This is
very similar to PostGIS BOX3D type:

a) BOX2Ds are 2D - BOX3D is 3D
b) BOX2Ds are represented by single-precision floating point numbers,
   while BOX3Ds are double-precision floating point numbers.
c) BOX2Ds will sometimes be slightly larger than you might expect.
   This is because the conversion from double-precision to 
   single-precision is inexact.  When this happens, the BOX2D will
   automatically expand itself.
   

Bounding Boxes
--------------

<this section for advanced users.  Ignore it if you don't understand
 what its saying.>

Bounding boxes are optional in LWGEOMs.  By not having one, you are
saving a small amount of space per LWGEOM geometry (16 bytes).

Bounding boxes are attached to any non-POINT geometry by default.

You can disable this automatic addition setting AUTOCACHE_BBOX to 0
in the top-builddir Makefile.config and read BBOXCACHE_BEHAVIOURS
for function-by-function behaviour explanation.

Manual control of bbox cache is available using DropBBOX() and AddBBOX().

UPDATE <table> SET <geomcol> = DropBBOX(<geomcol>);
UPDATE <table> SET <geomcol> = AddBBOX(<geomcol>);

Note that if you don't cache a BBOX spatial operators will be much slower,
as they'll need to compute all the boxes at every invokation.
If you use GiST indexes you will still note a slow down as a RECHECK
clause is specified for GiST operators. Also, GiST index updating
will be slower w/out a BBOX cache.


Space Saving
------------

LWGEOM indexes are approximately 40% smaller than PostGIS indexes.

A LWGEOM 'POINT(0 0)' takes up 21 bytes, a POSTGIS 'POINT(0 0)' takes
up 140 bytes.  This shows that LWGEOMs have a much smaller overhead.

LWGEOMs will store 2D points as 2 double precision numbers (16 bytes) -
PostGIS will store 2D points with 3 numbers (24 bytes).   This can be 
another big savings.