[postgis-users] ST_Union vs ST_Collect

[L Bogert-OBrien]

Hi,

Yes, thanks Jamie, that is indeed what I meant.  Didn't proofread the
message well enough!

Regards,

Loretta

----- Original Message -----
From: James DeMichele <james.demichele at redfin.com>
Date: Friday, September 10, 2010 6:50 pm
Subject: Re: [postgis-users] ST_Union vs ST_Collect

> Hello, Loretta, when you said: " replacing the ST_DUMP with
> ST_COLLECT...", did you mean that you replaced the ST_UNION with
> ST_COLLECT?
> 
> -Jamie
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of L
> Bogert-O'Brien
> Sent: Friday, September 10, 2010 3:26 PM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] ST_Union vs ST_Collect
> 
> The question:
> Why would CT_COLLECT work when CT_UNION does not?
> 
> The background:
> There are 49 distinct CMAs in my y2006.ca_ct.cbf_clp table.  Each of
> them have many multipolygons within them that touch each other by
> boundaries only.  I wanted to create a table containing a single
> dissolved multipolygon for each CMA, so I created the following query
> to insert the data into the new table, y2006.ca_cma_cbf_clp:
> 
> INSERT INTO y2006.ca_cma_cbf_clp (pruid, cmauid, the_geom_4269)
> (SELECT dmp.pruid, dmp.cmauid,
>         ST_MULTI(ST_BUFFER(ST_UNION(dmp.dmp_geom), 0.0))
>    FROM (SELECT pruid, cmauid,
>                 
> cleangeometry(ST_BUFFER((ST_DUMP(the_geom_4269)).geom,0.0)) AS 
> dmp_geom            FROM y2006.ca_ct_cbf_clp) AS dmp  --9683 rows 
> of dumped
> geometries
>    GROUP BY pruid, cmauid
>    ORDER BY pruid, cmauid);  --49 rows of CMAs
> 
> This returned 49 rows, but there were two of them that had geometries
> that were NULL, and the following errors were seen:
> 
> NOTICE:  TopologyException: found non-noded intersection between
> -79.4565 44.2272, -79.4565 44.2272 and -79.4565 44.2272, -79.4565
> 44.2272 -79.4565 44.2272
> NOTICE:  TopologyException: Directed Edge visited twice during
> ring-building -123.035 49.3916
> (These locations were within the two geometries that were missing.)
> 
> So I just made one change, replacing the ST_DUMP with ST_COLLECT, and
> then I got the results I was expecting.  There were no errors and all
> 49 entries in the new table had valid geometries.
> 
> Thanks for any clarification you can give on the difference between
> ST_UNION and ST_COLLECT and when one should be used over the other.
> 
> Regards,
> 
> Loretta
> 
> 
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