A little feedback of current findings for r.shaded.relief in GRASS6.4.2 in case anybody should have the same problem as we just had. We needed to create comparable results between the hillshade algorithm (ArcGIS) and the shaded.relief algorithm (GRASS) and compared the output. Values of shaded relief differ essentially and are generally smaller.
Based on http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z000000z2000000.htm I could correct the calculation to create the same results. Replacing the calculation in shaded relief with the syntax below creates an identical range of values. However, this can also been done through upscaling the output of r.shaded.relief with a factor of 2.549999108 (shaded.relief/hillshade=0.392157). Would it make sense to integrate this as an option?
The previous reported problem of negative values in the results are still existing. "if(cang<0,0,cang)" should remove those- doesn't it? Comments on this are very welcome.
patrick
r.mapcalc << EOF
$elev_out = eval( \\
x=($zmult*$elev[-1,-1] + 2*$zmult*$elev[0,-1] + $zmult*$elev[1,-1] \\
-$zmult*$elev[-1,1] - 2*$zmult*$elev[0,1] - $zmult*$elev[1,1])/(8.*ewres()*$scale) , \\
y=($zmult*$elev[-1,-1] + 2*$zmult*$elev[-1,0] + $zmult*$elev[-1,1] \\
-$zmult*$elev[1,-1] - 2*$zmult*$elev[1,0] - $zmult*$elev[1,1])/(8.*nsres()*$scale) , \\
slope=atan(sqrt(x*x + y*y)), \\
a=round(atan(x,y)), \\
a=if(isnull(a),1,a), \\
aspect=if(x!=0||y!=0,if(a,a,360.)), \\
zenith=90-$alt, \\
azimuth=360-$az+90, \\
cang=255*((cos(zenith)*cos(slope)) + (sin(zenith)*sin(slope)* cos($az-aspect))), \\
if(cang<0,0,cang), \\
if(isnull(cang), null(), cang))
EOF