[postgis-users] GEOS union() error

[Martin Davis]

Buffer always returns a polygonal answer (either a Polygon or a 
MultiPolygon).

Buffer will accept all inputs, including GeometryCollections.  The doc 
should be changed to reflect this.

Martin

Obe, Regina wrote:
> >From my experience it always returns a multipolygon giving a geometry
> collection of multipolygons.
>
> Actually collect doesn't always return geometry collections anyway.  If
> you say collect a bunch of polygons and they are not overlapping, then
> from my experience that returns a multipolygon.  If you start with
> multipolygons,then it returns a geometry collection, but if you then
> buffer that geometry collection, its seems to result in a multipolygon
> for me - never had it return a geometry collection.  I think its
> something about the process of buffering that strips out the geometries
> from the geometry collection - since if you feed it a single polygon
> encased in a multipolygon structure, it returns a polygon.  
>
> I don't know too much of the details of why that is.
>
> Thanks,
> Regina
>
>  
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Lee
> Keel
> Sent: Wednesday, November 07, 2007 9:33 AM
> To: PostGIS Users Discussion
> Subject: RE: [postgis-users] GEOS union() error
>
>
>
>   
>> -----Original Message-----
>> From: postgis-users-bounces at postgis.refractions.net
>>     
> [mailto:postgis-users-
>   
>> bounces at postgis.refractions.net] On Behalf Of Obe, Regina
>> Sent: Wednesday, November 07, 2007 8:22 AM
>> To: PostGIS Users Discussion
>> Subject: RE: [postgis-users] GEOS union() error
>>
>> Lee,
>>
>> Did you try the collect buffer?
>>
>> I'm not sure why we have that explicit in the documentation - could be
>> left over from another time? Should we remove that from the
>> documentation.
>>
>> I've done it and it works fine except when I had some invalid
>>     
> geometries
>   
>> I think it created some strange results but never errored out.
>>
>> Hope that helps,
>> Regina
>>
>>
>>     
> [Lee Keel] 
>
> I didn't try it, but I will if you say it will return reliable results.
> I
> was just unsure what the result set would be when using buffer on a
> geometry
> collection; will it return a geometry collection of polygons, or return
> one
> large multipolygon (which is what I am looking for)?
>
> I agree that if buffer() will accept a geometrycollection, then it
> should be
> removed from the documentation.  The same warning is in the
> documentation in
> a few places, is this the only place that it is valid now?  
>
> -LK
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022