[postgis-users] remove polygon from multipolygon

[Frank Durstewitz, Emporis GmbH]

Hi Kevin,

yes, that looks good. I was on that way, but could not figure out, how 
to get the geometry out of the dump rows.
Very nice this one, i have never seen this ".geom" (and still could not 
find it somewhere in the docs?).
Many thanks!

Kindly regards, Frank

Kevin Neufeld am 24.10.2008 20:44:
> 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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