Hi, Antonio. Despite their names, the QGIS "Union" geoprocessing tool has a completely different behavior than the PostGIS "ST_Union" function.
There is another QGIS tool called "Dissolve" which, when run without selecting an optional dissolve field, should have similar output to the ST_Union function. Keith On Wed, Jan 8, 2025 at 1:30 PM Antonio Valanzano via QGIS-User <qgis-user@lists.osgeo.org> wrote: > > Does someone know if this algorithm is based on GEOS? > 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). > > From QGIS help > > "This algorithm checks overlaps between features within the Input layer and > creates separate features for overlapping and non-overlapping parts. > > The area of overlap will create as many identical overlapping features as > there are features that participate in that overlap." > > > From PostGIS help "Aggregate variant: returns a geometry that is the union of > a rowset of geometries. The ST_Union() function is an "aggregate" function in > the terminology of PostgreSQL. > > That means that it operates on rows of data, in the same way the SUM() and > AVG() functions do and like most aggregates, it also ignores NULL geometries." > > > As Martin Davis wrote on the PostGIS user list following my message > > "The behaviour of ST_Union with polygons follows the mathematical concept > > of union in point-set topology. It produces as few polygons as possible > > which contain all the points of the input polygons...." > > ------------------------------------------------------------------------ > > Based on my little test the difference is that: > > QGIS algorithm removes the overlapping parts creating separate polygons with > the original attributes > > ST_Union does not remove any overlapping part (and a multistep procedure is > required to obtain the same results) > > Regards > Antonio _______________________________________________ QGIS-User mailing list QGIS-User@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
