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

[Nikos Alexandris]

Hi Frank. Could please help me a bit (...two questions below)

Frank Reekers wrote:
> 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

What is the projection that defines the Location here?

> 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

Searching for "Osnabrueck, Germany" in PGIS returns the coordinates:
52.267, 8.053

How were the coordinates transformed to metres (i.e. to 5794315.99, 
3435430.596)?
 
> Is there a reasonable explanation for this or did I something wrong?

Could it be an location error problem?
Kind regards, Nikos