[Qgis-developer] Intersection of two lines design

Bernhard Ströbl bernhard.stroebl at jena.de
Thu Mar 17 23:47:43 PDT 2016


I use QgsGeometry.intersection() and QgsGeometry.difference() in the 
DigitizingTools plugin (cutter/clipper) and I did never experience any 
errors in these functions no matter if the geometries share a vertex or 
not (neither during my tests nor during courses I gave nor in the bug 
tracker). So I wouldn't say this is generally true. I wonder, however, 
why it seems to be a problem in processing.


Am 17.03.2016 um 23:19 schrieb Maximilian Krambach:
> Hi,
> I've frequently stumbled upon the problem that several functions (like
> geomentry().intersection(geometry2), geometry().touches(geometry2) only
> return something useful if BOTH geometries have a vertex at the meeting
> point (which is inherited from GEOS)
> My expection: they do intersect or touch if they share space.
> I am Getting: Only a result if they share at least a vertex.
> This results in tools like geoprocessing:intersection or clip giving
> strange/unintuitive outputs.
> Is it a design decision or just never discussed? It could be solved
> quite easily, (see https://hub.qgis.org/issues/14504#note-2)
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