Dylan, What is your real SQL statement :) The below shouldn't even run e.g. you can't group by a table and you have to group by your difference since that is not an aggregate. Also I think you should be subtracting your non-buffered from your buffered if you want the doughnut left by your buffer.
I would expect your query to look like this SELECT A.country_name, ST_Difference(B.buffered_geom, A.buffered_geom) As diff_geom FROM A INNER JOIN B ON A.country_name = B.country_name Is your A geom really called buffered_geom too? Hope that helps, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Dylan Lorimer Sent: Sunday, May 11, 2008 9:57 PM To: PostGIS Users Discussion Subject: [postgis-users] ST_Difference (again) Hi All, So just as I thought I understood ST_Difference, I'm again confounded with some bizarre behaviour I can't figure out. I have a bunch of polygons all over the world that I've successfully grouped by country name. So for example, in India I have a single geometry that is a multipolygon, that consists of probably around 3000 random shaped polys in India. That is table A. I took those polygons and buffered them all by some amount. This produced...buffered polygons, as expected. That is table B. What I want to do now is remove the original polygons from the buffered poygons, and keep everything grouped by country, thereby leaving me the bit that I buffered by without the original. ST_Difference...right? Here's what I tried: SELECT A. country_name, ST_Difference(A.buffered_geom, B.buffered_geom) from A,B GROUP by A; What I get as the return value is A. Nothing more, nothing less. Any ideas? Cheers, dylan _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users ----------------------------------------- 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. _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
