On Wed, 17 Jun 2020 at 02:10, Richard Duivenvoorde <rdmaili...@duif.net> wrote: > > On 6/16/20 4:17 PM, Tudorache, Marian wrote: > > Hi everyone, > > > > I have a list of polygons given by a list of points. > > The polygons are properly drawn on QGIS canvas by creating the geometries, > > the each geometry is used to create a Qgsfeature which are saved on a > > shapefile. > > The problem appears in detecting if a point on the Earth is inside or > > outside the polygon. > > Using pyqgis QgsGeometry intersects function returns proper value. > > However when I export the polygon to intermaphics from Kongsberg Geospatial > > (former Gallium) sometime the point is inside other time is outside the > > polygon. > > I tried to switch the order of the points is QGIS ,but the intersects > > function always give me the same result regardless of the order of the > > points. > > In intermaphics the intersection between a polygon and a point varies with > > the order of the points which define a polygon. > > I talked to people from gallium and they confirmed the order of the points > > is important. > > In one direction a inner area of the polygon is the small surface and if I > > switch the order the inner area is the outside and it wraps the Earth on > > the opposite side. > > > > Does QGIS or pyqgis has a similar mechanism to determine which is the inner > > part of the polygon on a sphere or ellipsoid? > > Hi Marian, > > are'nt we here talking about the so called Right Hand Rule?
That's just a convention -- it doesn't change what the boundary actually represents. If you want to do analysis based on points which fall outside a digitized polygon, you should use a "disjoint" relationship. The alternative is to do what Nicolas suggested and make a polygon which covers the globe minus a small hole, but you'll get terrible performance with any analysis using that approach...! Nyall _______________________________________________ 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
