[postgis-users] Re: Testing for empty feature

[Sean Fulton]

ST_IsEmpty doesn't seem to work for me and I don't see the MultiPolygon 
being converted to a GeometryCollection.


civics=# SELECT astext(wkb_geometry) FROM scale150000 LIMIT 1;
         astext
------------------------
 MULTIPOLYGON(( EMPTY))
(1 row)

civics=# SELECT ST_IsEmpty(wkb_geometry) FROM scale150000 LIMIT 1;
 st_isempty
------------
 f
(1 row)

ST_NPOINTS works. It seems like there should be a more direct way.


	Sean

On 2008-07-04 12:03:33 -0400, Martin Davis <mbdavis at refractions.net> said:

> Actually JTS/GEOS handle empty MultiPolygons just fine. I don't know why
> the MULTIPOLYGON EMPTY is being converted to a general
> GeometryCollection - I think this must be something that is being done
> by PostGIS.
> 
> Seems like the answer is to use ST_IsEmpty. Doing text comparisons is
> always going to be risky.
> 
> Kevin Neufeld wrote:
>> I believe that an empty multipolygon is converted into an empty
>> geometrycollection in JTS and hence PostGIS.
>> 
>> 
>> postgis=# SELECT ST_AsText('MULTIPOLYGON EMPTY'::geometry);
>> st_astext
>> --------------------------
>> GEOMETRYCOLLECTION EMPTY
>> (1 row)
>> 
>> 
>> postgis=# SELECT ST_IsEmpty('MULTIPOLYGON EMPTY'::geometry);
>> st_isempty
>> ------------
>> t
>> (1 row)
>> 
>> Alternatively, I suppose you could get the number of points in the geometry.
>> 
>> postgis=# SELECT ST_NPoints('MULTIPOLYGON EMPTY'::geometry);
>> st_npoints
>> ------------
>> 0
>> (1 row)
>> 
>> 
>> Cheers,
>> Kevin
>> 
>> 
>> Sean Fulton wrote:
>> 
>>> How do I test for an empty feature such as 'MULTIPOLYGON(( EMPTY ))' ?
>>> 
>>> IsEmpty seems to work only for collections.
>>> 
>>> astext(wkb_geometry) = 'MULTIPOLYGON(( EMPTY ))' doesn't seem to work.
>>> 
>>> Thanks,
>>> 
>>> Sean
>>> 
>>> 
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