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

[Markus Neteler]

(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