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<title>[mapguide-users] OGR / GEOS selection problem</title>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi.<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I checked in a temporary fix to this problem. I simply warped one
of the coordinates of the input polygon by 10^-10. This is sufficient to make
OGR call GEOS to compute the exact intersection. Selection should work correctly
now in the AJAX viewer.<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I have not posted a compiled DLL yet, so Kenneth feel free to do
so, (-:<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Traian<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
mapguide-users-bounces@lists.osgeo.org
[mailto:mapguide-users-bounces@lists.osgeo.org] <b>On Behalf Of </b>Traian
Stanev<br>
<b>Sent:</b> Thursday, June 07, 2007 3:46 PM<br>
<b>To:</b> MapGuide Users Mail List; MapGuide Users Mail List<br>
<b>Subject:</b> RE: [mapguide-users] OGR / GEOS selection problem<o:p></o:p></span></p>
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<p class=MsoNormal>Hi Kenneth,<o:p></o:p></p>
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<p class=MsoNormal>Thanks for tracking this down. I would try to fix this by
bypassing Geos. FDO has utility code that can check for an intersection. We
could take the BBOX result and pass that through a real intersection check on
the provider side. This way the intersection check would be performed only on
geometries that are close enough to the query polygon so the overhead should
not be too bad.<o:p></o:p></p>
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<p class=MsoNormal>Traian<o:p></o:p></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-size:10.0pt'>-----Original
Message----- <br>
<b>From:</b> mapguide-users-bounces@lists.osgeo.org on behalf
of Kenneth, GEOGRAF A/S <br>
<b>Sent:</b> Thu 6/7/2007 2:54 AM <br>
<b>To:</b> MapGuide Users Mail List <br>
<b>Cc:</b> <br>
<b>Subject:</b> [mapguide-users] OGR / GEOS selection problem</span><o:p></o:p></p>
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<p><span style='font-size:10.0pt'>Maciej previously reported problems with the
OGR provider using PostGIS.<br>
<br>
The problem is with the OGR provider.<br>
Se the Trac ticket for details:<br>
<a href="http://trac.osgeo.org/mapguide/ticket/172">http://trac.osgeo.org/mapguide/ticket/172</a><br>
<br>
I have posted some follow up on the issue, but in hope someone knows a<br>
lot about GEOS, I'm also posting it here:<br>
<br>
<br>
I have been examining the problem, using a custom build of the OGR<br>
provider. It is true that without GEOS support in OGR, the Intersect<br>
only does a BBOX check.<br>
<br>
However, merely enabling GEOS will not work, as the viewer sends a<br>
square that is orthogonal to the axes (like a BBOX). The GEOS library<br>
will interpret this as another BBOX, and only perform the BBOX check<br>
anyway.<br>
<br>
When I change the polgyon sent from the viewer to a triangle, it takes<br>
22 seconds to calculate the Intersects result with a polygon with 8000<br>
linesegments. I have a 2+ GHz machine.<br>
<br>
It seems that GEOS always builds a full intersection matrix, before<br>
returning the result. This is obviously not optimal for a simple hit<br>
test. Even with this knowledge, I still don't see how it can take 22<br>
seconds to calculate 8000 * 3 intersections.<br>
<br>
Can someone explain this, or how to fix it?<br>
<br>
<br>
<br>
--<br>
Regards, Kenneth, GEOGRAF A/S<br>
<br>
<br>
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