<HTML dir=ltr><HEAD><TITLE>Re: [postgis-users] Collecting Polygons</TITLE>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>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.</FONT></DIV>
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<DIV dir=ltr><A href="http://postgis.refractions.net/pipermail/postgis-users/2008-June/020334.html">http://postgis.refractions.net/pipermail/postgis-users/2008-June/020334.html</A></DIV>
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<DIV dir=ltr>hope that helps,</DIV>
<DIV dir=ltr>Regina</DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> postgis-users-bounces@postgis.refractions.net on behalf of Lee Hachadoorian<BR><B>Sent:</B> Fri 6/27/2008 3:13 PM<BR><B>To:</B> PostGIS Users Discussion<BR><B>Subject:</B> Re: [postgis-users] Collecting Polygons<BR></FONT><BR></DIV>
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<P><FONT size=2>Interesting. Very clear now that you showed me the SQL. This was not<BR>really clear to me from the docs.<BR><BR>Thanks,<BR>--Lee<BR><BR>On Fri, Jun 27, 2008 at 3:00 PM, Kevin Neufeld <kneufeld@refractions.net> wrote:<BR>> It merges all geometries you supply it. If you supply geometries from<BR>> different tables, then yes.<BR>><BR>> SELECT ST_Union(geom)<BR>> FROM (<BR>> SELECT geom FROM mytable1 WHERE ...<BR>> UNION ALL<BR>> SELECT geom FROM mytable2 WHERE ...<BR>> ) AS foo;<BR>><BR>> -- Kevin<BR>><BR>> Lee Hachadoorian wrote:<BR>>><BR>>> Thanks, Kevin, that worked. I saw ST_Union, but I thought it was for<BR>>> joining polygons from different spatial tables. Does it do that also?<BR>>><BR>>> --Lee<BR>>><BR>>> On Fri, Jun 27, 2008 at 2:19 PM, Kevin Neufeld <kneufeld@refractions.net><BR>>> wrote:<BR>>>><BR>>>> Hi Lee,<BR>>>><BR>>>> If you want to merge several polygons together, dissolving them into a<BR>>>> single polygon, then you'll want to use ST_Union() instead of<BR>>>> ST_Collect().<BR>>>><BR>>>> ST_Union() - returns the point set union of all input geometries,<BR>>>> ST_Collect() - is a simple aggregate function that concatenates<BR>>>> geometries<BR>>>> together, ie a bunch of polygons to a multipolygon.<BR>>>><BR>>>> -- Kevin<BR>>>><BR>>>> Lee Hachadoorian wrote:<BR>>>>><BR>>>>> Upgraded to 1.3.3 to get rid of previous problem with ST_Collect, but<BR>>>>> I'm still not getting the output I want. What I want to do is take<BR>>>>> several polygons and mash them together into one polygon. The SQL I<BR>>>>> am using is:<BR>>>>><BR>>>>> SELECT ST_Collect(the_geom)<BR>>>>> FROM proj_mortgage.geo_hh_race<BR>>>>> WHERE pct_hispanic > 50;<BR>>>>><BR>>>>> But ST_Collect returns a GEOMETRYCOLLECTION, and I want a polygon or<BR>>>>> multipolygon. Do I need to do something with the GEOMETRYCOLLECTION<BR>>>>> returned by ST_Collect, or is ST_Collect the wrong function to use in<BR>>>>> the first place?<BR>>>>><BR>>>>> Thanks,<BR>>>>> Lee Hachadoorian<BR>>>>> PhD Student, Geography<BR>>>>> Program in Earth & Environmental Sciences<BR>>>>> CUNY Graduate Center<BR>>>>> _______________________________________________<BR>>>>> postgis-users mailing list<BR>>>>> postgis-users@postgis.refractions.net<BR>>>>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-users">http://postgis.refractions.net/mailman/listinfo/postgis-users</A><BR>>>><BR>>>> _______________________________________________<BR>>>> postgis-users mailing list<BR>>>> postgis-users@postgis.refractions.net<BR>>>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-users">http://postgis.refractions.net/mailman/listinfo/postgis-users</A><BR>>>><BR>>><BR>>><BR>>><BR>> _______________________________________________<BR>> postgis-users mailing list<BR>> postgis-users@postgis.refractions.net<BR>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-users">http://postgis.refractions.net/mailman/listinfo/postgis-users</A><BR>><BR><BR><BR><BR>--<BR>Lee Hachadoorian<BR>PhD Student, Geography<BR>Program in Earth & Environmental Sciences<BR>CUNY Graduate Center<BR>_______________________________________________<BR>postgis-users mailing list<BR>postgis-users@postgis.refractions.net<BR><A href="http://postgis.refractions.net/mailman/listinfo/postgis-users">http://postgis.refractions.net/mailman/listinfo/postgis-users</A><BR></FONT></P></DIV></BODY></HTML>
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