Hi Nyall, Just out of curiosity, would a very large polygon slow down a spatial query or would just a complex one? My testing with using shapefiles to reclassify LiDAR data did show that things could be sped up by splitting large polygons with a smaller grid but I had attributed this to the complexity of the polygons and not the size. Was this a wrong assumptions?
Nicolas Cadieux Ça va bien aller! > Le 16 juin 2020 à 18:21, Nyall Dawson <[email protected]> a écrit : > > On Wed, 17 Jun 2020 at 02:10, Richard Duivenvoorde <[email protected]> > 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 > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user _______________________________________________ Qgis-user mailing list [email protected] List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
