Antonio Valanzano anvalanz at gmail.com
Wed Jan 8 10:29:59 PST 2025
Does someone know if this algorithm is based on GEOS?
Hi Antonio,
the geometric operations executed by the "Union" processing algorithm in
QGIS are actually performed mostly by the GEOS library.
You can see the algorithm source at
https://github.com/qgis/QGIS/blob/master/src/analysis/processing/qgsalgorithmunion.cpp
I am asking this because its *behaviour is* different from that of ST_Union
in PostGIS or Spatialite (both based I assume on the GEOS library).
Indeed, while the name is similar, the descriptions and the behaviour of
the "Union" processing algorithm and the "ST_Union" function are totally
different (kind of the opposite).
Have you already read
https://gis.stackexchange.com/questions/316269/st-union-in-postgresql-and-qgis
and https://groups.google.com/g/spatialite-users/c/VCvyIumpdfs ?
See also how the "Union" tool work in ESRI ArcMap
https://desktop.arcgis.com/en/arcmap/latest/tools/analysis-toolbox/how-union-analysis-works.htm
Regards.
Andrea Giudiceandrea
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