On Wed, 17 Jun 2020 at 20:27, Nicolas Cadieux <[email protected]> wrote: > > Hi Nyall, > > Just out of curiosity, would a very large polygon slow down a spatial query > or would just a complex one?
That's an open question. It depends on lots of different factors, including crucially what you're comparing the large polygon against. But a general answer would be "yes": a large but simple polygon does have the potential to result in a worse query vs a smaller polygon using a disjoint relationship. > 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? I think in general you'll find that polygon complexity is a much more significant factor vs the actual size, so your conclusion is fine. (My hint: using the QGIS Processing "subdivide" or PostGIS' st_subdivide tools are perfect to help in this situation) Nyall > > 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
