Hi Grasshoppers,
I am not sure what kind of curvature Tim is refering to but here is
an r.mapcalc statement to calculate Terrain Shape Index (McNab, W.H. 1989.
Terrain shape index: Quantifying effect of minor landforms on tree height.
Forest Sceince, 35:91-104)
tshape=((1000*(dem[-1,-1]-dem[0,0])+\
1000*(dem[-1,0]-dem[0,0])+\
1000*(dem[-1,1]-dem[0,0])+\
1000*(dem[0,1]-dem[0,0])+\
1000*(dem[1,1]-dem[0,0])+\
1000*(dem[1,0]-dem[0,0])+\
1000*(dem[1,-1]-dem[0,0])+\
1000*(dem[0,-1]-dem[0,0]))/8)/plot radius
the 1000 is to compensate for GRASS non-floating point operation...
McNab's TSI = (((Z1-Z0)+ (Z2-Z0) + ... + (Zn-Z0))/N)/R
Z1,Z2,...,Zn = elevations sampled around the plot boundary
Z0 = elevation at plot center
N = number of observations
R = plot radius (this will be a factor of the resolution of your dem )
Don Catanzaro
dgc@cast.uark.edu