[postgis-users] How to tell if 2 geometries are spatially equal

Obe, Regina robe.dnd at cityofboston.gov
Fri May 30 13:16:06 PDT 2008

I recall this having come up before.  I always thought that ~= would
tell me if 2 geometries are spatially equal but it doesn't seem to.

The only way I can figure to determine spatial equality is if 

ST_Within(A,B)  And  ST_Within(B,A)  (or ST_Difference(A,B) AND
ST_Difference(B,A) both return an empty geometry collection)

--So case in point

SELECT geom1 ~= geom2 as what, ST_Within(geom1, geom2) AND
ST_Within(geom2, geom1) As spatial_equal,
	ST_AsText(ST_Difference(geom1, geom2)) as diffgeom12,
ST_AsText(ST_Difference(geom2, geom1)) as diffgeom21 
FROM (SELECT 'LINESTRING(1 1, 1 2, 1 3)'::geometry As geom1, 
	'LINESTRING(1 1, 1 3)'::geometry As geom2) As foo


 what | spatial_equal |        diffgeom12        |        diffgeom21

Is there a function / operator that does that (also what does geom1 =
geom2 compare - is it just bounding boxes or is that the spatially equal
operator I am looking for?)


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