land area on sloping surfaces

[Bill Brown]

As far as I can tell, this calculation is more an art than science
since it's heavily dependent on data resolution, but I have a
simple program I call r.surf.area that basically turns a raster
dataset into a terrain by connecting the centers of cells and making 2
planar triangles out of each square, then adds up the 3d area of the 
triangles.   It only works if using same units for elevation as for
e-w res (i.e. meter elevation in UTM).

If you want to try it, let me know & I'll mail you the code (only 
about 250 lines).  I don't really want to put it on incoming since there's
no documentation and I don't have any way of knowing what kind of
accuracy it has.  It should be OK for comparing relative "roughness"
of terrain, though.

- Bill
brown at ginko.cecer.army.mil