Sorry for the barrage of questions lately! But suddenly PostGIS is becoming extremely useful to my day-to-day activities. So congrats on a fantastic product that is letting me (thus far) skip on purchasing any ESRI software.
Questions: 1) Given N polygons in a table called A, and another table of country borders, how do I generate a set of polygons that represent where the N polygons of table A do NOT exist? Sort of a mask of the polygons in A clipped by a country border. Actually, I know how to clip by borders, so I'm really just looking for how to find the area not occupied by polygons in A. 2) I know this one has been beaten to death on this list, but yet in the archives I still can't exactly find what I want. I've got absurd amounts of polygons, millions. But for the sake of argument let's just say I have a lot. I've also got country borders. I can easily find which polys fall within certain countries, but what I want to do is then generalize those polys that overlap into single polygons. It appears as if ST_Union is my candidate, but I can't see to get it working. Do I need to join against the same table? Here's what I've so far come up with but that I don't think is correct: select ST_Union(a.the_geom,b.the_geom) as the_geom from the_table a, the_table b where ST_Intersects(a.the_geom, b.the_geom); Any thoughts on this would be helpful. Cheers! -dylan _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
