land area on sloping surfaces

[Grass package]

r?9??95?08?13?36?am at bnr.ca>
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> 
> In mountainous terrain on a sloping surface the planimetric land area
> is less than the actual surface area on the ground.  Is there an easy
> way to use a DEM in GRASS to find the actual surface area?  Maybe I'm
> asking for a 3-D GIS...
> 
>       Bill
>       bakerwl at uwyo.edu
> 
Interesting problem. It should be possible to use r.mapcalc to look for four
neighboring DEM measurements, calculate distances and angles along the 
surface between them, and the surface area of the enclosed lozenge from
that. This would create a new map, which can then be used to derive
aggregate surface measurements a la r.average.

        Martijn
        p.m.van-leusen at bham.ac.uk