Dylan, So are you saying that with ST_Union you got topological intersection errors but with ST_MemUnion you didn't? This would be very intriguing if that were the case since in theory both should yield the same result albeit slightly differently. ST_Union does an array collect and then unions where as ST_MemUnion keeps on unioning 2 geometries at a time. Thanks, Regina
________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Dylan Lorimer Sent: Monday, May 05, 2008 9:47 PM To: [email protected] Subject: [postgis-users] Re: ST_Difference Perplexes Me! Wanted to follow-up on this with the SQL that worked in the end. Firstly...thanks so much Regina! I've no confidence that I ever would have figured out this sql on my own, and yet looking at it now it makes perfect sense! So here it is in case anyone else is interested: - vmap_countries contains country boundaries - published_both_prototype_union contains dissolved polygons all over the world (millions of them.) CREATE TABLE not_meter_or_less_prototype AS SELECT ST_Difference(vmap_countries.the_geom, C.the_sum_geom), vmap_countries.country_na FROM vmap_countries INNER JOIN (SELECT ST_MemUnion(published_both_prototype_union.the_geom) As the_sum_geom, vmap_countries.country_na FROM vmap_countries INNER JOIN published_both_prototype_union ON ST_Intersects(vmap_countries.the_geom, published_both_prototype_union.the_geom) WHERE vmap_countries.country_na IN ('China', 'India', 'Japan') GROUP BY vmap_countries.country_na ) As C ON vmap_countries.country_na = C.country_na; Many many (many) thanks to those on the list that have helped me out! Cheers, dylan ----------------------------------------- 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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