Hi Marco, On 28/07/2015 10:38, Marco Hugentobler wrote: > Hi > > While working on QgsDistanceArea, I noticed the definition of perimeter > in QGIS is not the same as in PostGIS. PostGIS returns the length of > outer and inner rings, while QGIS expects the length of the outer ring > only: > > test_distancearea.py: > > polygon = QgsGeometry.fromPolygon( > [ > [ QgsPoint(0,0), QgsPoint(3,0), QgsPoint(3,3), > QgsPoint(0,3), QgsPoint(0,0) ], > [ QgsPoint(1,1), QgsPoint(2,1), QgsPoint(2,2), > QgsPoint(1,2), QgsPoint(1,1) ], > ] > ) > > perimeter = da.measurePerimeter(polygon) > assert perimeter == 12, "Expected:\n%f\nGot:\n%f\n" % (12, perimeter) > > > Using the following in PostGis returns 16 (length of inner and outer > rings): > > SELECT ST_Perimeter( ST_GeomFromText( 'POLYGON(( 0 0, 3 0, 3 3, 0 3, 0 > 0),(1 1, 2 1, 2 2, 1 2, 1 1))', -1 ) ); > > What is your opinion? Is it okay to change it in QGIS to also consider > the length of inner rings or are there any strong arguments against that?
Good question ! :) I don't know if there is one ideal definition of the perimeter of inner rings. After all, we may decide to see the inner perimeter as a negative one ... as it is for the area. So, just a proposition: let the user decide what to do with the two perimeters (outer and inner), adding an option to retrieve separately the inner perimeter ? _______________________________________________ Qgis-developer mailing list Qgis-developer@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-developer
