[GRASS-user] r.proj or r.resamp.rst issues

[Glynn Clements]

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 <glynn at gclements.plus.com>