[GRASS-user] global radiation calculated with r.sun seems to be very high

[Frank Reekers]

the global radiation, calculated with r.sun, seems to be very high in 
comparison to PVGIS (Photovoltaic Geographical Information System).
 - r.sun for 14. Feb: ~2363 Wh/m² per day
 - PVGIS: 1330 Wh/m² per day Irradiation on horizontal plane for 
february on average

Below the detailed steps I did in GRASS and with PVGIS:
1. PVGIS:
a) http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php -> search for 
location 'Osnabrueck, Germany' -> choose monthly radiation -> click 
calculate -> the output table shows 1330 Wh/m² Irradiation on horizontal 
plane for february.

2. GRASS
a) Take an existing DEM with the resolution of 1m and set all z-values 
to 68m to have a completely horizontal plane:
GRASS 6.4.0RC6 (GRASS)> r.mapcalc dsxx94c68=dsxx94c at PERMANENT*0+68.00

b) Set Region
GRASS 6.4.0RC6 (GRASS)> g.region rast=dsxx94c68

c) Create slope & aspect:
GRASS 6.4.0RC6 (GRASS)> r.slope.aspect elevation=dsxx94c68 at PERMANENT 
slope=sdsxx94c68 aspect=adsxx94c68 --overwrite

d) Run r.sun for day 45:
GRASS 6.4.0RC6 (GRASS)> r.sun elevin=dsxx94c68 at PERMANENT 
aspin=adsxx94c68 at PERMANENT slopein=sdsxx94c68 at PERMANENT 
glob_rad=g45phxx94c68  day=45 --overwrite

e) Get global radiation:
GRASS 6.4.0RC6 (GRASS)> r.what input=g45phxx94c68 
east_north=3435430.596,5794315.99
3435430.596|5794315.99||2362.9213867188


Is there a reasonable explanation for this or did I something wrong?

Thanks for your help
Frank
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