[postgis-users] Followup - Re: Intersection problems with complex polygons, (TECHER Jean David)
Andreas Neumann
neumann at karto.baug.ethz.ch
Thu Jan 13 01:53:58 PST 2005
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/
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