[postgis-users] Question on INTERSECTION functionality...

[Aaron Racicot]

Thanks, I will give this a try and play with the filter.  I will post
back to the group when I have some results!

Aaron

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Carl
Anderson
Sent: Monday, March 21, 2005 7:25 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Question on INTERSECTION functionality...

Aaron Racicot wrote:

<snip>

> The problem I am having is that while all of my initial polygons are 
> MULTIPOLYGON type, the intersection operator returns 
> GEOMETRYCOLLECTION for some of the intersections. I read over the 
> simple features spec and it seems to imply that some intersections 
> will generate shapes that can not be represented anymore as polygons, 
> but instead polygons and other types like lines. Is this true?
>
yes difference, symdifference, and intersection may all return geometry 
types of other dimensions or
collections of various geometry types.

> If it is, I have a problem in that I can not do the intersection 
> operator on GEOMETRYCOLLECTION and therefore can not continue my 
> process of breaking down overlapping polygons. Is there any solution 
> to this problem? I was hoping that you could do some type of "cast" 
> from GEOMETRYCOLLECTION to another type, say MULTIPOLYGON, and get a 
> "best" approximation by dropping other geometry types out of the 
> collection and just returning the resulting polygons.
>
As long as you desire to ignore the geometries of other dimensions
(
point and multipoint are geometries of dimension 0
linestring and multilinestring are geometries of dimension 1
polygon and multipolygon are geometries of dimension 2
)
you can install and use the attached function to filter geometries.

This

C.