[GRASS-user] how to use cloud cover with r.sun?
Markus Neteler
neteler at itc.it
Fri Aug 25 07:52:50 EDT 2006
(cc dev list)
On Fri, Aug 25, 2006 at 04:55:34PM +0530, Vishal Mehta wrote:
> Thanks everyone,
> a few thoughts-
> 1. Neither the r.sun manual, nor the r.slope.aspect manual, states that
> the x,y and elevation units should be the same. In fact both manuals say
> the elevation should be in meters! r.sun talks about how if the maps are
> projected then it does'nt matter- but it does not talk of use with
> lat-long system(nor does r.slope.aspect).
>
> do these manuals then just assume that we are using a projection where the
> x,y are in meters??
...
> I still cant get over the idea that GRASS cant handle correctly slope
> generation in lat-long coordinates with elevation in meters!
No, this isn't fully the case:
r.slope.aspect help
...
zfactor Multiplicative factor to convert elevation units to meters
But for r.sun we need to find a solution.
Markus
> thanks again,
> vishal
>
> On 8/25/06, Hamish <[1]hamish_nospam at yahoo.com > wrote:
>
> > 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??
>
> Yes, it is possible. Maybe even likely.
>
> > And can you please tell me where the 1852*60 comes from?
>
> There are 1852 meters in a nautical mile. There are 60 nautical miles in
> a degree of latitude. elev_in_meters / (1850.0 * 60) = elevation in deg
>
> The width of a deg longitude is equal to width_lat*cos(lat).
>
> If rescaling the elevation into degress be sure to /(1852.0*60). This
> ensures that the result will be a floating point value (as opposed to
> /(1852*60) ).
>
> > I guess i would have to creat a new location in UTM coordinates and
> > then do the projecting back and forth you suggest..
>
> I think this is the saner solution, but I expect that a single UTM zone
> is not an appropriate projection for working with an area the size of
> India.
>
> Markus wrote:
> > This should be a projection covering the entire subcontinent
> > in a reasonable way - what are the official projection(s) for
> > that area?
>
> If you have problems finding one, you might ask for help on the PROJ.4
> mailing list.
>
> Hamish
>
> References
>
> Visible links
> 1. mailto:hamish_nospam at yahoo.com
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--
Markus Neteler <neteler itc it> http://mpa.itc.it/markus/
ITC-irst - Centro per la Ricerca Scientifica e Tecnologica
MPBA - Predictive Models for Biol. & Environ. Data Analysis
Via Sommarive, 18 - 38050 Povo (Trento), Italy
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