[postgis-users] geomunion HOWTO?
randre at gmail.com
Thu Mar 13 14:41:25 PDT 2008
I'm trying to find a way to generate "dissolved" geometries without
exporting shapefiles from PostGIS and performing the operating in ArcGIS. I
found some instructions online at
These work fine on their example, but the opeartion when applied to my data
set never completes. I realize my data set is pretty large (), but the same
dissolve operation when done via ArcGIS on a shapefile exported by pgsql2shp
takes around 5 minutes to complete. This leads me to believe I'm doing
something completely wrong, and I would love to get some feedback from those
of you with experience doing this. Below are the steps I've done.
Step 1 - create a "crop_3" table that contains only crop3 values, and
a class. This completes within 30 secs:
create table "test_suit_h_crop3_class" (
"alloc_id" char(8) PRIMARY KEY,
insert into "test_suit_h_crop3_class" ("alloc_id", "crop3", "class",
select vw_suit_area_h.alloc_id, vw_suit_area_h.crop3,
when crop3 < 1 then 'class_0'
when crop3 >= 1 and crop3 < 860 then 'class_1'
when crop3 >= 860 and crop3 < 1720 then 'class_2'
when crop3 >= 1720 and crop3 < 3440 then 'class_3'
when crop3 >= 3440 and crop3 < 5160 then 'class_4'
when crop3 >= 5160 and crop3 < 6880 then 'class_5'
when crop3 >= 6880 and crop3 < 7740 then 'class_6'
when crop3 >= 7740 then 'class_7'
end AS class,
Step 2 - create a temp "dissolve" table to store the results of a
geometric union run of the above table, grouped by "class". I run out of
patience before this ever completes (I've let it run for hours.)
CREATE TABLE "test_suit_area_h_crop3_diss" (
gid serial PRIMARY KEY,
INSERT INTO "test_suit_area_h_crop3_diss" (the_geom,class)
SELECT astext(multi(geomunion(the_geom))) AS the_geom, class FROM
"test_suit_h_crop3_class" GROUP BY class;
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