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

[Tim Keitt]

On Fri, Feb 20, 2015 at 2:05 PM, Jonathan Greenberg <jgrn at illinois.edu>
wrote:

> 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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But slope is linear, so just take the ratio dz/dx (in one dimension --
slightly more involved in 2d as I assume you want the slope along the
steepest gradient). You just need the distance between grid cells and the
elevation change.

THK

-- 
http://www.keittlab.org/

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