[gdal-dev] OGR and GEOS

Kevin Webb kfw4 at cornell.edu
Mon Dec 22 14:26:24 EST 2008

At 12:46 PM 12/22/2008, Ragi Y. Burhum wrote:
>Date: Mon, 22 Dec 2008 09:37:15 -0500
>From: Kevin Webb <<mailto:kfw4 at cornell.edu>kfw4 at cornell.edu>
>Subject: [gdal-dev] OGR and GEOS
>To: <mailto:gdal-dev at lists.osgeo.org>gdal-dev at lists.osgeo.org
><<mailto: at postoffice7.mail.cornell.edu> at postoffice7.mail.cornell.edu>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>The documentation for the OGRGeometry class mentions the GEOS library in a
>few of the
>method definitions, but I wonder if there is a "stronger" OGR dependency on
>the GEOS
>library than the documentation suggests. Perhaps GEOS should be a required
>I had done some processing using OGRGeometry::Intersects() without the GEOS
>installed, I then installed GEOS to run the same tests using
>After installing GEOS, but before running the OGRGeometry::Distance()
>tests, I re-ran the
>code using OGRGeometry::Intersects() and noticed different intersection
>Before-GEOS, the Intersects() method indicated intersection with 2 layer
>polygons. After-GEOS,
>the Intersects() method indicated intersection with 1 layer polygon. Note:
>After-GEOS results
>for both Intersects() and Distance() == 0 were the same.
>Should the GEOS library be a requirement for deterministic OGR processing?
>Thank you.
>With GEOS and without GEOS you will get "deterministic" results. The 
>difference is that the algorithm to calculate intersections without GEOS 
>is simpler - just bounding box intersections. You want GEOS installed.
>- Ragi

Thank you for your reply Ragi.

You are correct, I should not have used to word "deterministic;" with-GEOS 
or without-GEOS I produced the same output
result for each processing run.

As Frank suggested in his post, I think more clarity is needed in the 
write-ups for the methods that use alternate
processing logic with-GEOS installed when compared to a similar operation 
without-GEOS installed.


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Kevin Webb
Cornell Lab of Ornithology
159 Sapsucker Woods Rd.
Ithaca, NY 14850
Tel: 607.254.2103
Fax: 607.254.2415
kfw4 at cornell.edu
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