[Qgis-developer] Approximation and geoprocessing

Paolo Cavallini cavallini at faunalia.it
Fri Jan 10 06:01:54 PST 2014

Hi all.
I got into an interesting problem, before opening a few tickets I'd like
to discuss a line of action.
1. Some sw (namely geomedia) produces shapefiles with very slight
differences (same layer, modified and exported several times); I suppose
this is due to truncation/approximation in coordinate precision.
2. As a result, the diff between these layers produces a number of
virtually 0 width polygons, and some topological errors.
3. This in turn leads to 2 problems:
3.1. subsequent analyses fail because of the errors, and the user has no
clue about the reasons
3.2. The microareas are tricky and time consuming to remove (if they are
isolated, a possible solution is to select and remove those smaller than
an arbitrary threshold, but if they are linked to some "real" polygons,
this will not work).
I would therefore suggest a few improvements over the toolchain, i.e.:
* add a parameter in analyses, to either snap original data within a
threshold before running the analysis, or to clean up the results afterwards
* check & warn the user of topo errors in source layers, and of possible
wrong results; maybe this should be optional, as this will slow down the
analysis, and may be useless after first cleanup.

Sample data available for those interested.

All the best.
Paolo Cavallini - www.faunalia.eu
QGIS & PostGIS courses: http://www.faunalia.eu/training.html

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