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
