[postgis-users] Multi centroids

[Kevin Neufeld]

Yes.  Use ST_Dump to explode your MULTI* object, take the ST_Centroid of each, and ST_Collect them together again.

SELECT ST_AsText(ST_Collect(the_geom))
FROM (
   SELECT ST_Centroid((ST_Dump(the_geom)).geom) AS the_geom
   FROM (
     SELECT
       'MULTIPOLYGON (
         (( 0 0, 0 1, 1 1, 1 0, 0 0 )),
         (( 2 2, 2 3, 3 3, 3 2, 2 2 ))
        )'::geometry AS the_geom
   ) AS foo
) AS foo2;

           st_astext
-----------------------------
  MULTIPOINT(0.5 0.5,2.5 2.5)
(1 row)


Cheers,
Kevin

Mike Toews wrote:
> Hi,
> 
> Is there a simple way to extract the multiple centroids as MULTIPOINT 
> geometries from MULTI* objects (i.e., ST_MULTICENTROID(geometry) )? The 
> ST_CENTROID function only returns POINT geometries.
> 
> (An example application of this is to initially place CAD labels for 
> MULTIPOLYGON features, since it appears that AutoCAD is not smart enough 
> to place these properly, and need manual adjustment to avoid label 
> conflicts. I have always been baffled by the purpose of multipoints 
> until now.)
> 
> I'm guessing this isn't a built-in function and the solution will be a 
> custom SQL or PL/pgSQL function. Thanks for any help/ideas.
> 
> -Mike
> 
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