Thanks Glynn. That makes sense now. I did not know about the inherent problem of overshooting in cubic interpolation. ASTER DEMs I have seem to be pretty consistent in terms of data ranges and representation of values (based on r.report), so I am not sure about the noise effect in my case. However, does this mean that I am sort of out of "corrective" options if I want to use cubic reprojection of imagery from Latlong to UTM? I mean I either use nearest method for error-free reprojection or I have to be careful about the results of cubic method. Finally, when you say "use higher values for tension parameter", I guess you mean values lower than default 40. such as 30. or 20.
Bulent On Fri, May 7, 2010 at 5:08 AM, Glynn Clements <gl...@gclements.plus.com>wrote: > > 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> > -- BÜLENT ARIKAN School of Human Evolution and Social Change Arizona State University Tempe - AZ 85287-2402
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