HELP DEM Calculations

[Paul Maxwell Brett]

        I have been using grass4.1 (binary release) and was doing
some numerical elevation modeling.  What I want to do is determine
where in the slope i am located, i.e. upper slope, mid slope, or
lower slope.
        I was told that could be done with grass.  If anyone knows 
a way in which I could accomplish this your input would be greatly
appreciated.
                                        Paul M. Brett
                                        GEOIDAL 
                                        Memorial University
                                        pbrett at bart.geog.mun.ca