[GRASS-user] [GRASSLIST:1160] RE: Surface area of a DEM

[Carlos "Guâno" Grohmann]

Jonathan,

You can do that with r.mapcalc.

Calculations can be made with r.mapcalc, using the following formula
for a 50m regular grid:

50*(sqrt((exp((tan(A)*50), 2)) + 2500))
Where A is the slope map.


from: GROHMANN, C.H. 2004. Morphometric analysis in Geographic
Information Systems: applications of free software GRASS and R.
Computers & Geosciences, 30 (9-10):1055-1067.


cheers

Carlos Grohmann


On 7/20/06, Hamish <hamish_nospam at yahoo.com> wrote:
> Jonathan Greenberg wrote:
>
> > I was thinking this, but does the fact that a DEM grid cell is square
> > cause complications in this formulation?  A 45 degree slope to the
> > west would have a different surface area than a 45 degree slope to the
> > southwest.  I've come across some solutions that use a set of 8
> > triangles created by the lines between the center cell and each of its
> > 8 neighbors to help in the calculation, but I was hoping this ability
> > might be buried in a GRASS command someplace.
>
>
> think of each cell as a spinning dinner plate on a stick.
>
> you can rotate each cell in the direction of the aspect, and the
> slope->projected area will be the same.
>
> dx*dy is its own entity, or if you prefer, the r.mapcalc command doesn't
> know about the aspect.
>
> ??
>
> Hamish
>
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-- 
+-----------------------------------------------------------+
              Carlos Henrique Grohmann - Guano
  Geologist M.Sc  - Doctorate Student at IGc-USP - Brazil
Linux User #89721  - carlos dot grohmann at gmail dot com
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