[GRASS-user] r.sun vs r.sunmask

[Markus Neteler]

Patrick,

On Fri, Oct 19, 2012 at 5:50 PM, Patrick S. <patrick_GIS at gmx.ch> wrote:
> r.sun -s elevin=r_dem incidout=r_sun_shadow aspin=r_aspect slopein=r_slope
> day=80 time=17 --o

I would leave the slope and aspect calculation to the module.
Be sure to set the computational region properly before running the
module:

# check if the resolution and extent are as desired, note that in
# mountainous areas the region must be larger to catch all
# shadows, use r.los or r.viewshed to get an idea about that:

g.region rast=r_dem -p

r.sun -s elevin=r_dem incidout=r_sun_shadow day=80 time=17

r.mapcalc "shadows = if (r_sun_shadow == 0, 1, null() )"

No idea about the gradient you reported, you need to tell us more about
your study area and its size.

Markus