[postgis-users] Collecting Polygons
robe.dnd at cityofboston.gov
Fri Jun 27 12:42:43 PDT 2008
Actually you can still get MULTIPOLYGONS with ST_Collect and ST_Collect tends to perform better the ST_Union, but doesn't dissolve. If you just need MULTIPOLYGONS and no dissolving you might be better off with ST_Collect and dump. As described below.
hope that helps,
From: postgis-users-bounces at postgis.refractions.net on behalf of Lee Hachadoorian
Sent: Fri 6/27/2008 3:13 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Collecting Polygons
Interesting. Very clear now that you showed me the SQL. This was not
really clear to me from the docs.
On Fri, Jun 27, 2008 at 3:00 PM, Kevin Neufeld <kneufeld at refractions.net> wrote:
> It merges all geometries you supply it. If you supply geometries from
> different tables, then yes.
> SELECT ST_Union(geom)
> FROM (
> SELECT geom FROM mytable1 WHERE ...
> UNION ALL
> SELECT geom FROM mytable2 WHERE ...
> ) AS foo;
> -- Kevin
> Lee Hachadoorian wrote:
>> 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?
>> On Fri, Jun 27, 2008 at 2:19 PM, Kevin Neufeld <kneufeld at refractions.net>
>>> 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_Union() - returns the point set union of all input geometries,
>>> ST_Collect() - is a simple aggregate function that concatenates
>>> 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?
>>>> Lee Hachadoorian
>>>> PhD Student, Geography
>>>> Program in Earth & Environmental Sciences
>>>> CUNY Graduate Center
>>>> postgis-users mailing list
>>>> postgis-users at postgis.refractions.net
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
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PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
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