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

[Paul Ramsey]

If ST_Contains() is returning both polygons then there's probably
something wrong with the forest polygon (it doesn't have a hole, or it
has both a polygon that has a hole and another component that *is* the
hole, etc).

P

On Tue, May 5, 2009 at 9:19 AM, Martin Davis <mbdavis at refractions.net> wrote:
> 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
>
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