[GRASS-user] Re: [GRASSLIST:1159] Surface area of a DEM

[Hamish]

Jonathan Greenberg wrote:

> I'd like to estimate the surface area of each cell in a DEM
> (r.surf.area seems to just give you the value for the entire dem, no?)
> -- by surface area I am referring to the "true" ground area as opposed
> to the planar projected ground area (which is constant for all cells)
> -- flat areas the area = the planar projected area (e.g. a 30m cell
> that is flat has 900m2 of surface area, but an inclined slope will
> have > 900m2).  Is there a grass module to do this, or any
> recommendation on how to calculate this?  Thanks!


1) Use r.slope.aspect to make a slope map

2) r.mapcalc "area = nsres()*ewres() / cos(slope)"


then r.univar's sum for the area map should give an answer close to
r.surf.area?


It's a fractal problem, so use results with care.


Hamish