Le mercredi 22 janvier 2014 23:14:01, John Abraham a écrit : > Please help me with something potentially important. The documentation for > gdaldem slope says “If the horizontal unit of the source DEM is degrees > (e.g Lat/Long WGS84 projection), you can use scale=111120 if the vertical > units are meters” but I don’t think this works very far away from the > equator. I think the algorithm assumes that the north-south units are the > same as the east-west units, whereas obviously away from the equator > degrees of longitude become a lot shorter. > > I’m looking at my attempt at avalanche slope mapping, and seem to see > east-west slopes showing up with higher slopes than north-south slopes > with identically spaced contour intervals. > > Am I right? Or am I seeing things?
Yes, the -scale 111120 only works for approximate visual rendering, not for something life critical. > > If I’m right, perhaps: > > 1) Someone should fix the documentation, so that people don’t rely on > incorrect slope calculations for something life-critical such as avalanche > slope mapping, and If you want to provide an update of the doc, its source is in apps/gdal_utilities.dox ( http://svn.osgeo.org/gdal/trunk/gdal/apps/gdal_utilities.dox ) > > 2) Someone could tell me a better solution, I think perhaps the best idea > is to just resample the grid into a projection based on meters, before > applying gdaldem without any scaling factor. Yes that would probably be certainly more appropriate. For example in the UTM zone of your data, or potentially some more fitted grid. > > Thanks for your help. This has been bugging me for sometime, but I finally > decided to speak up. > > — > John Abraham > j...@hbaspecto.com -- Geospatial professional services http://even.rouault.free.fr/services.html _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev