[postgis-users] remove polygon from multipolygon

[Kevin Neufeld]

What about something like this:

SELECT ST_Collect(geom)
FROM (
   SELECT (ST_Dump(geom)).geom
   FROM my_multi_polygon_table) AS foo
WHERE ST_Y(ST_Centroid(geom)) <= 0;

Explode the multipolygon into individual polygons using ST_Dump, and 
ST_Collect the ones back up again where the centroid falls within some 
specification.

-- Kevin

Frank Durstewitz wrote:
> Hello list,
> 
> i must remove from multipolygon some polygons, for example remove all 
> polygons from "russia", where latitude of centroid of polygon is <= 0 
> (in this case the part of russia  which is near alaska).
> 
> I think this should be possible (ST_Dump...), but i cannot find any way 
> to do so.
> Deleting the vertices by hand via a desktop gis is painfull slow and 
> will take ages - i tried this and need ~ 5 minutes for a very small 
> island of ~ 20 points... (i found no desktop gis where i can select 
> multiple vertices by a bounding box and delete them at once).
> 
> Thanks,
> 
> Frank
> 
> ST_Summarize gives
> 
> MultiPolygon[BS] with 1393 elements
>  Polygon[] with 1 rings
>   ring 0 has 15 points
> ...
>  Polygon[] with 1 rings
>   ring 0 has 25 points
> 
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