Unfortunately there's not, Robert, because a negative elevation
means the sun is below the horizon and a positive one means it's
above: those are different positions. There may be a workaround:
tilt the DEM! (After all, this is what is really happening,
isn't it: the earth it tilting relative to the sun; the sun is
not moving through the sky...) Because DEMs assume all heights
are perfectly vertical, this is an approximation, but it could
be effective provided you don't have to tilt more than a few
degrees. Here's one way to do it. Let the sun's bearing be 'a'
degrees east of north (e.g., the sun at the southwest will have
a bearing around 225 degrees). Let the desired amount of tlt be
t degrees. Find the coordinates of a point near the middle of
the DEM; you don't have to be precise. Let these coordinates be
(x,y). With a calculator or the equivalent, compute the three
numbers T = tan(t)(to seven significant figures) and then
compute S = sin(a) C = cos(a) b = x*S + y*C.The tilted DEM is
obtained by adding th