Patrick S. patrick_GIS at gmx.ch
Tue Oct 23 04:44:06 PDT 2012

```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
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

```