[postgis-users] Collecting Polygons

[Lee Hachadoorian]

Thanks, Kevin, that worked.  I saw ST_Union, but I thought it was for
joining polygons from different spatial tables.  Does it do that also?

--Lee

On Fri, Jun 27, 2008 at 2:19 PM, Kevin Neufeld <kneufeld at refractions.net> wrote:
> Hi Lee,
>
> If you want to merge several polygons together, dissolving them into a
> single polygon, then you'll want to use ST_Union() instead of ST_Collect().
>
> ST_Union() - returns the point set union of all input geometries,
> ST_Collect() - is a simple aggregate function that concatenates geometries
> together, ie a bunch of polygons to a multipolygon.
>
> -- Kevin
>
> Lee Hachadoorian wrote:
>>
>> Upgraded to 1.3.3 to get rid of previous problem with ST_Collect, but
>> I'm still not getting the output I want.  What I want to do is take
>> several polygons and mash them together into one polygon.  The SQL I
>> am using is:
>>
>> SELECT ST_Collect(the_geom)
>> FROM proj_mortgage.geo_hh_race
>> WHERE pct_hispanic > 50;
>>
>> But ST_Collect returns a GEOMETRYCOLLECTION, and I want a polygon or
>> multipolygon.  Do I need to do something with the GEOMETRYCOLLECTION
>> returned by ST_Collect, or is ST_Collect the wrong function to use in
>> the first place?
>>
>> Thanks,
>> Lee Hachadoorian
>> PhD Student, Geography
>> Program in Earth & Environmental Sciences
>> CUNY Graduate Center
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-- 
Lee Hachadoorian
PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center