Hi,<div><br></div><div>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.</div>
<div><br></div><div>Since I interpolate GDEMs from 30m to 10m using 'r.resamp.rst' (keeping tension at 40.), I need to fix this problem. I thought of making negative value cells "null" but this may create more problems during interpolation (actually, I tried and r.resamp.rst could not complete interpolation). If I take absolute values of cells to make negative cells positive, then I introduce cells with unrealistic elevation data. It seems like after interpolation –without any correction of negative values–, the number of negative value cells do not increase, they are still less than 1% of total cells. </div>
<div><br></div><div>I will appreciate any suggestions why cubic method produces negative values in some imagery and how to fix this before or after interpolation.</div><div><br></div><div>* I did not try changing default r.resamp.rst values, such as tension, because most of the imagery I interpolated have very accurate results; the elevation difference between interpolated (10m res) and original (30m res) is around 1 meter.</div>
<div><br></div><div>Thank you for your comments.</div><div> </div><div>-- <br>BÜLENT ARIKAN<br>School of Human Evolution and Social Change<br>
Arizona State University<br>Tempe - AZ<br>85287-2402<br>
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