[GRASS-user] [GRASSLIST:1178] how to use cloud cover with r.sun?

[Vishal Mehta]

Hamish,

You may be right that elevation in meters is being used as degrees (my
region is lat long, elevation in meters)...because since my last email I
have run the r.sun module using a new region based on the finer GTOPO30
resolution. With the shadow option on, i am getting even more zeros- almost
all zeros!!

But is it really possible that the r.sun module (i'm using Grass 6.0.1 on
Ubuntu Breezy) would treat  elevation in degrees??

And can you please tell me where the 1852*60 comes from?
I guess i would have to creat a new location in UTM coordinates and then do
the projecting back and forth you suggest..

Jose, I am not using a particular model for PET, rather I am using a bunch
of equations mostly from FAO-56 (just google it, the book is online); met.
data from the Climate Research Unit, UK.
And there's no easy way to do it, at least based on my research. To give
just 2 examples,
1. r.sun module gives solar radiation(clear sky), and you need observed
spatio-temporal data (difficult to get) on either clear sky index or the
coeffd, coefb parameters to get real sky radiation;
2. The FAO 56 manual does have some simple options. but these involve
working with extra terrestrial radiation- i dont think r.sun gives that (its
internally calculated i believe).

anyway..it might take me 2-3 months to produce some final output..

cheers,
vishal
On 8/24/06, Hamish <hamish_nospam at yahoo.com> wrote:
>
>
> I think you are correct, elevation in meters is probably being treaded
> as degrees, so you get an unwanted 1852*60 vertical exaggeration.
> Sounds plausible at least.
>
> Solutions:
>
> a) reproject your DEM into a meter-based projection (I couldn't tell you
> which one), then reproject the results back to lat/lon.
>
> b) rescale your DEM into "degrees". e.g.:
>
> r.mapcalc elevmap_scaled="elevmap / 1852*60"
>   [maybe figure in cos(mean_lat)]
>
> That will probably mess up the linke turbidity functions which I believe
> takes physical elevation into account. (but I could be wrong)
>
>
> Hamish
>
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