Description

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

There are 3 variants of this. The one that takes a geometry will return a POLYGON geometry representation of the bounding box and is the most commonly used variant.

ST_Expand is similar in concept to ST_Buffer except while buffer expands the geometry in all directions, ST_Expand expands the bounding box an x,y,z unit amount.

Units are in the units of the spatial reference system in use denoted by the SRID

	Pre 1.3, ST_Expand was used in conjunction with distance to do indexable queries. Something of the form the_geom && ST_Expand('POINT(10 20)', 10) AND ST_Distance(the_geom, 'POINT(10 20)') < 10 Post 1.2, this was replaced with the easier ST_DWithin construct.

	Bounding boxes of all geometries are currently 2-d even if they are 3-dimensional geometries.

Examples

	Examples below use US National Atlas Equal Area (SRID=2163) which is a meter projection

--10 meter expanded box around bbox of a linestring
SELECT CAST(ST_Expand(ST_GeomFromText('LINESTRING(2312980 110676,2312923 110701,2312892 110714)', 2163),10) As box2d);
					 st_expand
------------------------------------
 BOX(2312882 110666,2312990 110724)

--10 meter expanded 3d box of a 3d box
SELECT ST_Expand(CAST('BOX3D(778783 2951741 1,794875 2970042.61545891 10)' As box3d),10)
							  st_expand
-----------------------------------------------------
 BOX3D(778773 2951731 -9,794885 2970052.61545891 20)

 --10 meter geometry astext rep of a expand box around a point geometry
 SELECT ST_AsEWKT(ST_Expand(ST_GeomFromEWKT('SRID=2163;POINT(2312980 110676)'),10));
											st_asewkt
-------------------------------------------------------------------------------------------------
 SRID=2163;POLYGON((2312970 110666,2312970 110686,2312990 110686,2312990 110666,2312970 110666))

		

See Also

ST_AsEWKT, ST_Buffer, ST_DWithin, ST_GeomFromEWKT,ST_GeomFromText, ST_SRID