extend r.slope.aspect to compute the values at the edges

[hellik]

taken from the ML

​http://lists.osgeo.org/pipermail/grass-dev/2015-January/072779.html

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.

it would be nice that r.slope.aspect compute the values at the edges, at least as an option. I agree that any reasonable estimate is better than the current shrinking region.