[GRASS-dev] r.slope.aspect: one pixel less in the output at the border shrinks region extent in further calculations

Helena Mitasova hmitaso at ncsu.edu
Sun Jan 4 06:36:27 PST 2015


you can use r.resamp.rst which computes slope and aspect and does not shrink the region. But you may need to adjust the parameters
to make sure it works well.

Another way would be to modify r.slope.aspect to compute the values at the edges - a second order polynomial min.square approximation
is used to estimate the derivatives in r.slope.aspect (you get the well known differencing function when you do the math) and it can be used
also to compute the values at the edge cells, but I had no luck convincing others that it is the right thing to do.
I believe that any reasonable estimate is better than the current shrinking region (in r.flow we just propagate the same values to the edges),
but that does not seem to be the consensus. 
Also, implementation  for the edges is not straightforward because of how
GRASS works with rows, but a smart developer could certainly do it (you can compute the edge values e.g.in r.mapcalc).

I hope this clarifies it and may be helps a little bit. I am quite interested in the flat valley bottoms - here in NC they are often
found as bottoms of abandoned, dried out mill ponds.


Helena Mitasova
Professor at the Department of Marine, 
Earth, and Atmospheric Sciences
and Center for Geospatial Analytics
North Carolina State University
Raleigh, NC 27695-8208
hmitaso at ncsu.edu
"All electronic mail messages in connection with State business which are sent to or received by this account are subject to the NC Public Records Law and may be disclosed to third parties.” 

On Jan 4, 2015, at 7:54 AM, Helmut Kudrnovsky wrote:

> Hi Pietro,
>> this is not a bug. 
> I unterstand the same. my question is more about how to implemented the
> mentioned iterative algorithm which uses coarser resolution by each step and
> how to avoid an extent shrinking of e.g. about 400m with the coarsest
> resolution.
> -----
> best regards
> Helmut
> --
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