[GRASS-user] Request r.sun

[Alois Simon]

Dear all,
I tried out the tool r.sun and r.sun.daily for calculation of the global 
irradiance at around a latitude of 47.2° N
Since the calculated values (Wh/m²/day) appeared very high to me, I 
tried the same calculation with ArcGIS10 Tool Area Solar Radiation (with 
default settings).
The values were always higher with GRASS GIS and very different from 
ArcGIS10, from 4 times higher at day=1 (winter) to 1.5 times higher at 
day=180 (summer). Thereby the results from ArcGIS10 seems to be more 
realistic.
The calculated duration of insolation (insol_time) were the same for 
both tools (-> geographic location is correct).
Also both tool should calculate global radiation with clear sky conditions.
Since the differences of the calculated values are greater during winter 
time (lower sun angle), it seems that it has something to do with the 
atmospheric transmittivity. I also tried different "linke_value" (within 
the range given in the documentation), with only minor changes of values.

Can anybody help me with this problem?
Do I forgot some important settings?
code sample: /r.sun elevation=dgm_test1 at PERMANENT 
aspect=dgm_test1_aspect at PERMANENT slope=dgm_test1_slope at PERMANENT 
beam_rad=beam_rad_001 diff_rad=diff_rad_001 refl_rad=ref_rad_001 
glob_rad=glob_rad_001 linke_value=3 day=1 --overwrite/
Are there any other values, except "linke_value", by which I can 
influence the transmittivity?

Many thanks in advance.

Sincerely,
Alois Simon

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