[geos-devel] Small gaps between results of GEOSDifference?

Marco Hugentobler marco.hugentobler at sourcepole.ch
Fri Feb 15 04:02:11 PST 2013

Hi all

I have two geometries (A, B). Now I'm doing GEOSDifference(B, A) and 
then a union of A and the result of the difference operation. The result 
is different compared to GEOSUnion(A, B).
Shouldn't GEOSUnion(A,B) be equal to GEOSUnion( A, GEOSDifference(B, A) 
) ? I have attached a simple test program to reproduce the issue. 
Probably I'm doing something wrong in the code?

Background of my question is that when digitising in QGIS, people are 
frequently using the option to avoid intersection between polygons 
(which calls GEOSDifference). If they repeat that, it turns out, there 
are small gaps between the results of the difference operation and the 
newly added polygons have a wrong shape 
(http://hub.qgis.org/issues/2921). Doing a union of the difference 
results shows the same small gaps betwen the polygons.

Thanks for any hints,

Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee

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