[postgis-users] Re: GeometryCollection Intersection

[Martin Davis]

Yes, that's correct, the geometry relationship predicates are not 
available for GeometryCollections.  The reason for this is that because 
GC's have very loose semantics (e.g. polygons can cross one another 
being the main one) it's not possible to robustly & exactly evaluate 
some predicates in certain situations. 

I guess intersection can always be evaluated correctly, however.  With 
the new work on prepared geometry, we perhaps could look at widening the 
scope a bit. (This might even be supported right now - I can't remember 
offhand).

Markus Schneider wrote:
> Hello again,
>
> actually I wanted to ask about the "intersects" operator (and not
> "intersection") which is used in the given example query.
>
> Markus Schneider wrote:
>   
>> Hello,
>>
>> is it correct that geometric operations (especially intersection) are not
>> available if one of the geometries is a GeometryCollection?
>>
>> My postgis_full_version() is:
>>
>> "POSTGIS="1.3.1" GEOS="2.2.3-CAPI-1.1.1" PROJ="Rel. 4.4.9, 29 Oct 2004" USE_STATS"
>>
>> When I try to execute the following statement:
>>
>> SELECT X1.GEOM,X1.NAME,X1.ID FROM MULTITEST X1
>> WHERE X1.GEOM && SetSRID('BOX3D(174259.098 487844.529, 177282.329
>> 484821.298)'::box3d,28992)
>> AND intersects (X1.GEOM, SetSRID('BOX3D(174259.098 487844.529, 177282.329
>> 484821.298)'::box3d,28992))
>>
>> I get the error message:
>>
>> ERROR: Relate Operation called with a LWGEOMCOLLECTION type.  This is unsupported
>> SQL Status:XX000
>>
>> I already digged the source code a bit and found the apparent source of the
>> message in lwgeom_geos_c.c. It's the check errorIfGeometryCollection(...).
>>
>> Is there any chance to perform an intersection between a Box and a
>> GeometryCollection anyway? Or are there plans to implement this?
>>
>> Thanks in advance,
>>     
>
>
> Best regards,
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022