How have you failed? Did you get an error or some unexpected result. ST_Difference takes as input 2 geometries - ST_Difference(geom1, geom2) and returns that portion of geom1 that is not in geom2. I generally use it in conjunction with intersection. If 2 geometries are Disjoint - do not intersect then ST_Difference returns just geom1 I believe which is not terribly interesting and a bit of a waste fo processing speed since there are easier ways to return that. So you would use it something like this - for example if you wanted to know that portion of a land parcel that is not built on, then you would do something like SELECT ST_Difference(l.the_geom, b.the_geom) FROM parcels l INNER JOIN buildings b ON ST_Intersects(l.the_geom, b.the_geom) Note here I am making the wrong assumption that only one building lies on a parcel - so this example is a bit flawed (but simpler than my real query), but hopefully will give you an idea of how it is used. Note: There is the less popular ST_SymDifference(geom1, geom2) that returns that portion of geom1 + geom2 that is not shared. Thus it is called Symmetric Difference since the order of the arguments does not matter. Hope that helps, Regina
________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Richard Heimann [C] Sent: Monday, October 22, 2007 3:47 PM To: [email protected] Subject: [postgis-users] ST_Difference Syntax PostGIS users, I have failed at constructing the ST_Differnece operation. Can someone expose the proper syntax? Thanks Richard ----------------------------------------- The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer.
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