[postgis-users] Problem with ST_Within, Polygons and Multipolygons

[Martin Davis]

Polygons do not contain their holes.  For a point P in a MultiPolygon M 
which is inside a hole in one of the component Polygons you should have:

ST_within(P, M) = false
ST_contains(M, P) = false

Best to post your WKT and your query to confirm that's the problem.

Jorge wrote:
> Hello,
>
> I have a question. If a multipolygon has inner "holes", are these holes
> part of the multipolygon? I mean, if I want to check if a given point is
> inside of a multipolygon (using "within" or "contains") and this point
> is in the hole, should "within" and "contains" return "true"?
>
> Being more specific, I have a multypolygon that represents a forest, in
> a map. This multipolygon has a hole inside. Then, I have another
> multipolygon that fits exactly into the hole, and represents an airport.
> If I select a point from the airport, this point is inside the second
> multipolygon, of course, but should it be inside the first one too?
> Because my problem is that I have to decide if the point is part of the
> airport or part of the forest, and "within" and "contains" methods
> return both multipolygon, instead of only the second one (the airport).
>
> Maybe is more clear with an image:
> http://www.nebulared.com/tmp_geo/multipolygon.jpg
>
> Thanks in advance!
>
> Regards
> Jorge
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>
>   

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