[mapguide-users] OGR / GEOS selection problem

[Kenneth, GEOGRAF A/S]

What version of the GDal and GEOS did you use?

Regards, Kenneth, GEOGRAF A/S




Traian Stanev skrev:
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> I just tried it on my Australia states data set which has polygons 
> with 16000+ vertices. It works fine and it is not slow (less than 1 
> second from mouse click to the selected polygon being highlighted in 
> the AJAX viewer). May be it is a problem in your configuration. FWIW 
> my data came from a SHP file.
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> Traian
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> *From:* mapguide-users-bounces at lists.osgeo.org 
> [mailto:mapguide-users-bounces at lists.osgeo.org] *On Behalf Of 
> *Kenneth, GEOGRAF A/S
> *Sent:* Tuesday, June 12, 2007 10:36 AM
> *To:* MapGuide Users Mail List
> *Subject:* Re: [mapguide-users] OGR / GEOS selection problem
>
>  
>
> Yes, I have tried doing the selection with a triangle, and it also 
> works, but there is another bug in GEOS, that makes it EXTREMELY slow 
> for any geometries with +2000 points.
>
> I have posted some info on trac about it:
> http://trac.osgeo.org/mapguide/ticket/172
>
> So, the original problem is "fixed", but it opens another problem, 
> which may have more to do with GEOS than with the provider.
> I think the related speed problem makes the provider more useless with 
> the fix than without.
>
> I can build a new provider, but I don't have the setup for Oracle and 
> PostGIS.
>
>
> Regards, Kenneth, GEOGRAF A/S
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> Traian Stanev skrev:
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> Hi.
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> 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.
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> I have not posted a compiled DLL yet, so Kenneth feel free to do so, (-:
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> Traian
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> *From:* mapguide-users-bounces at lists.osgeo.org 
> <mailto:mapguide-users-bounces at lists.osgeo.org> 
> [mailto:mapguide-users-bounces at lists.osgeo.org] *On Behalf Of *Traian 
> Stanev
> *Sent:* Thursday, June 07, 2007 3:46 PM
> *To:* MapGuide Users Mail List; MapGuide Users Mail List
> *Subject:* RE: [mapguide-users] OGR / GEOS selection problem
>
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> Hi Kenneth,
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> 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.
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> Traian
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>     -----Original Message-----
>     *From:* mapguide-users-bounces at lists.osgeo.org
>     <mailto:mapguide-users-bounces at lists.osgeo.org> on behalf
>     of Kenneth, GEOGRAF A/S
>     *Sent:* Thu 6/7/2007 2:54 AM
>     *To:* MapGuide Users Mail List
>     *Cc:*
>     *Subject:* [mapguide-users] OGR / GEOS selection problem
>
>     Maciej previously reported problems with the OGR provider using
>     PostGIS.
>
>     The problem is with the OGR provider.
>     Se the Trac ticket for details:
>     http://trac.osgeo.org/mapguide/ticket/172
>
>     I have posted some follow up on the issue, but in hope someone knows a
>     lot about GEOS, I'm also posting it here:
>
>
>     I have been examining the problem, using a custom build of the OGR
>     provider. It is true that without GEOS support in OGR, the Intersect
>     only does a BBOX check.
>
>     However, merely enabling GEOS will not work, as the viewer sends a
>     square that is orthogonal to the axes (like a BBOX). The GEOS library
>     will interpret this as another BBOX, and only perform the BBOX check
>     anyway.
>
>     When I change the polgyon sent from the viewer to a triangle, it takes
>     22 seconds to calculate the Intersects result with a polygon with 8000
>     linesegments. I have a 2+ GHz machine.
>
>     It seems that GEOS always builds a full intersection matrix, before
>     returning the result. This is obviously not optimal for a simple hit
>     test. Even with this knowledge, I still don't see how it can take 22
>     seconds to calculate 8000 * 3 intersections.
>
>     Can someone explain this, or how to fix it?
>
>
>
>     --
>     Regards, Kenneth, GEOGRAF A/S
>
>
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