[postgis-users] GEOS union() error

Obe, Regina robe.dnd at cityofboston.gov
Wed Nov 7 06:40:54 PST 2007


>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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