Bulent Arikan wrote: > I am running GRASS 6.5 svn (Snow Leopard). I have several ASTER GDEMs > (Latlong, 30m res.), which I reprojected into UTM using both 'nearest' and > 'cubic' methods ('r.proj'). Only in some imagery that are reprojected in > cubic, I ended up having couple of cells (literally, 1-2 cells out of 8 > million in average) with minus (-) values. For example, in a DEM where the > elevation values are between 800-2600 meters, I have cell values between > -150 and -85 meters. This does not seem to be an issue in reprojected > imagery with the nearest method. I am not sure how these minus values are > introduced at the first place.
Cubic interpolation can introduce overshoot, as can other forms of spline interpolation. Linear and nearest-neighbor interpolation don't have this issue. With r.resamp.rst, the problem can be alleviated to a degree by using higher values for the tension= parameter. Also, if your data is noisy, this will tend to exaggerate the gradients, making overshoot more likely. Filtering the data first will reduce the errors. -- Glynn Clements <gl...@gclements.plus.com> _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user