[R-sig-Geo] Proper algorithms for computing slopes for global/polar DEMs?

[Jonathan Greenberg]

r-sig-geo'ers:

We're trying to figure out a good algorithm for computing topographic slope
for Alaska, which is both huge and near the pole.  Our DEM is currently in
geographic coordinates, but we need slope over a fixed distance (change in
elevation vs. horizontal meters).  The issue is, there is no projection to
switch this to to get an accurate set of slope calculations.  Of course
with degrees, the X and Y distances change over the globe -- so the
horizontal distances will be variables from one window to the next.  I have
seen various solutions published, but I wanted to see if someone knows of a
decent coded-up algorithm for this.  Thoughts?

Some background:
http://gis.stackexchange.com/questions/14750/how-to-use-srtm-global-dem-for-slope-calculation/40464#40464

--j

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