[postgis-users] Followup - Re: Intersection problems with complex polygons, (TECHER Jean David)

[TECHER Jean David]

Hi

"When zooming in the original GIS (ESRI Arcmap) I found out that the two
small holes (the red and blue thickly stroked polygons) touch each
other. That might explain the problem."

U're right...with the images I wanted to send to you I have the same
think...

Perhaps as U said maybe a bug in postgis...I will think about the problem
before this w-k
----------------------------------------------------------------
TECHER Jean David
Responsable Informatique 01MAP
e-mail: davidtecher at yahoo.fr
Bureau: 04 67 45 60 27
Portable: 06 85 37 36 75
site perso : http://techer.pascal.free.fr/postgis/
site pro: http://www.01map.com/download/
K-S:"The greatest trick the devil pulled off was convincing people he didn't
exist"
------------------------------------------------------------
----- Original Message -----
From: "Andreas Neumann" <neumann at karto.baug.ethz.ch>
To: <postgis-users at postgis.refractions.net>
Sent: Thursday, January 13, 2005 10:53 AM
Subject: [postgis-users] Followup - Re: Intersection problems with complex
polygons, (TECHER Jean David)


> Salut Jean-David,
>
> Thank you for answering and sorry my late reply.
>
> >Hi Andreas
> >
> >I'm a friend of Bernd :-)
> >
> >
> yes, I know you you "virtually" through Bernd and André ;-)
>
> >If you make a zoom in the intersection area you have two disjoint
polygons
> >in this intersection ...just see the attached images...
> >
> >
>
> I could not see the attached image (I get the mails in a digest and the
> link in the archives did not correctly display) - could you please mail
> it to me privately? (neumann at karto.baug.ethz.ch)
>
> I made my own investigations on the problem meanwhile. You can find the
> situation in the following two files:
>
> query: http://www.carto.net/neumann/temp/intersection_test.sql.gz
> visualization of the result:
> http://www.carto.net/neumann/temp/intersection_result.svg
>
>
> Explanation of the visualization of the problem:
>
> The green polygon is the input polygon, the red-stroked/unfilled
> rectangle is the intersect polygon.
>
> The result is a geometry collection containing two lines and three
> polygons. AsSVG cannot display geometry collections, so I manually split
> the result using astext and fed the individual parts to AsSvg. Anyway,
> I'd expect when intersecting a polygon with a polygon, that I'd receive
> a multipolygon as a result and not a geometry collection containing also
> linestrings.
>
> The visualized result in
> http://www.carto.net/neumann/temp/intersection_result.svg shows the
> correctly intersected part (blue filled polygons) and the two
> linestrings I got in the resulting geometry collection with a thicker
> red and blue stroke.
>
> So - I still think that this is a bug in Postgis/Geos. Unless I
> misunderstood what the "intersection" function actually does.
>
> When zooming in the original GIS (ESRI Arcmap) I found out that the two
> small holes (the red and blue thickly stroked polygons) touch each
> other. That might explain the problem.
>
> Any help on that situation is appreciated. Meanwhile I can fix the bug
> by changing the input geometry a little bit, but for the long run I
> would appreciate having a better solution than having to change the
> original geometry.
>
> Thanks and all the best,
> Andreas
>
> --
> --
> ----------------------------------------------
> Andreas Neumann - Department of Cartography
> Swiss Federal Institute of Technology (ETH)
> ETH Hoenggerberg, CH-8093  Zurich, Switzerland
> Phone: ++41-1-633 3031, Fax: ++41-1-633 1153
> e-mail: neumann at karto.baug.ethz.ch
> www: http://www.karto.ethz.ch/neumann/
> SVG.Open: http://www.svgopen.org/
> Carto.net: http://www.carto.net/
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users