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<p>Dear all,<br>
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 <br>
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). <br>
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. <br>
The calculated duration of insolation (insol_time) were the same
for both tools (-> geographic location is correct). <br>
Also both tool should calculate global radiation with clear sky
conditions. <br>
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. <br>
</p>
<p>Can anybody help me with this problem? <br>
Do I forgot some important settings? <br>
<font size="-2">code sample: </font><font face="Arial"><i><font
size="-2">r.sun elevation=dgm_test1@PERMANENT
aspect=dgm_test1_aspect@PERMANENT
slope=dgm_test1_slope@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</font></i></font><br>
Are there any other values, except "linke_value", by which I can
influence the transmittivity?<br>
</p>
<p>Many thanks in advance.</p>
<p>Sincerely,<br>
Alois Simon<br>
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