[GRASS-user] r.sun annual radiation

Hamish hamish_b at yahoo.com
Mon Sep 5 23:13:34 EDT 2011

Salvatore wrote:
> thanks for the script, it works. I want to be sure that the
> result is the global radiation map expressed in W/m2/yr (or
> it is W/pixel/yr?). I compare it with ones made with ArcGis
> and the values are differents. 

how much different? just by a few percent or wildly different?

I have just done some back-of-the-envelope calculations based
on Spearfish's latitude on Sept 21st and the solar constant,
both instantaneous at 12 noon, and a rough integration over the
day, and both answers of my very crude calcs come within 3% and
8% of the presumably much more correct r.sun results. (and I'd
expect the atm scattering to take more out in the dawn/dusk
hours so that increase to 8% diff for the daily sum doesn't
worry me) So they would seem to me to be in the correct order of

It is not surprising that ESRI is using a different method/
algorithm than r.sun uses, and if so not surprising that the
answer would be slightly different. Can you hold the albedo and
atmospheric turbidity identical in both methods?

the basic integration of flux from a point source following a
curve for a known geometry should be rather similar, but the
fancy atmospheric and ground albedo adjustments is where I'd
expect to see the differences and interesting science.

> Another question…the albedo effect is excluded?

the albedo in r.sun is either given by a raster map containing
ground coefficients, or the "alb" parameter, which defaults
to 0.2.


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