[Qgis-user] geoalgorithm "Union"

[Andrea Giudiceandrea]

> 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