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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