[Community] Intersects problem

[Mario Ceresa]

Hello Pascal,
thanks for your prompt response! Now I see why using the polygon or
its exterior is different: it makes much more sense. Still I confirm
that all the intersections of both the linear ring and the polygon
with the points in your example return True. Contains works as
expected.

I'm wondering whether there is any known bug in the geos library.
Could you please let me know if you are using the 3.1 too or a
different version?

Thanks a lot for your help

Mario



On Thu, May 7, 2009 at 8:42 PM, Pascal Leroux <pa.leroux at gmail.com> wrote:
> Hi Mario,
>
> I'm surprised with your results. That's what I get with Shapely 1.0.12 :
>
>>>> from shapely.geometry import Point
>>>> from shapely.geometry.polygon import LinearRing
>>>> test = LinearRing(((1,1),(2,3),(4,2),(3,0)))
>>>> points = [ Point((2,2)),Point((1,0.5)),Point((1.5,0.5)) ]
>>>> map(test.intersects,points)
> [False, False, False]
>>>> map(test.contains,points)
> [False, False, False]
>
> But, don't forget that a LinearRing instance is a 1D object/shape.
> So your "test" LinearRing is different from a Polygon whose outer ring is
> "test" :
>
>  >>> from shapely.geometry import Polygon
>>>> poly = Polygon(((1,1),(2,3),(4,2),(3,0)))
>>>> map(poly.intersects,points)
> [True, False, False]
>>>> map(poly.contains,points)
> [True, False, False]
>
> Pascal
>
> 2009/5/7 Mario Ceresa <mario.ceresa at torrescalla.it>
>>
>> Hello everybody,
>> I'm sorry to ask a very stupid question, but I don't undestand well
>> how intersects and contains work:
>>
>> >> test = LinearRing([(1,1),(2,3),(4,2),(3,0)])
>> >> points = [Point(2,2),Point(1,0.5),Point(1.5,0.5)]
>> >> map(test.intersects,points)
>> returned [True, True, True] and I was expecting [True,False,False]
>> >> map(test.contains,points)
>> returned [False, False, False] and I was expected [True,False,False]
>>
>> It may be that I'm thinking at the wrong geometry but why (1;0.5) and
>> (1.5;0.5) are considered intersected with the polygon if they are
>> outside? Is the intersection done with the bounding box of the
>> objects?
>>
>> Is there any way to retrieve the points contained within the polygon?
>>
>> I'm using Shapely-1.0.12-py2.5 and geos 3.1 on a x86_64 Fedora 9
>>
>> Thanks and regards,
>>
>> Mario
>> _______________________________________________
>> Community mailing list
>> Community at lists.gispython.org
>> http://lists.gispython.org/mailman/listinfo/community
>
>
> _______________________________________________
> Community mailing list
> Community at lists.gispython.org
> http://lists.gispython.org/mailman/listinfo/community
>
>