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@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
Are there any other values, except “linke_value”, by which I can influence the transmittivity?
Many thanks in advance.
Sincerely,
Alois Simon