On Monday 25 August 2008, Dylan Lorimer wrote: > Hi PostGIS Folks, > > I have a table with several polygons, some overlapping, each arbitrarily > sized, and some falling across the border of France and Spain. For the sake > of my question, it doesn't really matter the dimensions, number, or > complexity of the polygons, but to keep things simple let's say that I have > 50 of these polygons in table A. > > I also have polygons representing France and Spain, stored in table B with > country_name as the ... country name. > > The goal of this exercise is to compute the sum of all unique coverage by > polygons over each country. So the first thing I need to do is dissolve the > polygons where they intersect to get rid of overlap. Let's say I do this > using ST_MemUnion and use the political borders in table B as follows: > > CREATE TABLE dissolved_polygons AS SELECT ST_MemUnion(a.the_geom), > b.country_name FROM a LEFT JOIN b ON ST_Intersects(a.the_geom, b.the_geom) > > Now table dissolved_polygons has 2 entries - one for france and one for > spain. Each is a multi-polygon representing the coverage in each country. > However, for dissolved polygons that span the border, they are fully > represented in each country. My question at this point is how I can then > delete any portion of the polygon in France that extends into Spain, and > vice-verca? > > Perhaps there is a better way altogether to do this? Maybe use > ST_Difference on the original non-dissolved polygons? > > I hope my question is clear enough. > > Many Thanks. > -dylan
Hi Dylan, I am not sure if I understand your problem-- it may help to actually tell us what you are doing. From what I gather, something like this may work: -- pseudo-code c = intersection(a,b) d = GeomUnion(c) group by country_name Make sense? Cheers, Dylan PS: that looks a lot like a "note to self"! -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
