aspect and drainage direction maps

xavi at xavi at
Thu Nov 16 07:00:00 EST 1995


I am wondering how are aspect and drainage direction maps generated.
I thought that simpler direction maps, as those generated by r.watershed
or the agnps-type option of, were build with the 3x3 matrix 
surrounding any cell. So, every cell would be oriented to the one
with the lowest value among the eight cells around it. I imagined that the
aspect map generated by r.slope.aspect would take into account more cells,
cause it is able to make 360 orientation categories. 
After analyzing drainage direction maps generated by r.watershed or
I realized that the algorithm doesn't look to be so easy. 
Anybody could explain me how this kind of algorithms work?
Thanks a lot!

                Xavier Ros
        Unitat de Geodinamica Externa i Hidrogeologia
        Universitat Autonoma de Barcelona.
        e-mail: xavi at 

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