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

[Bulent Arikan]

Hi,

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.

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.

I will appreciate any suggestions why cubic method produces negative values
in some imagery and how to fix this before or after interpolation.

* 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.

Thank you for your comments.

-- 
BÜLENT ARIKAN
School of Human Evolution and Social Change
Arizona State University
Tempe - AZ
85287-2402
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