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

[Dylan Beaudette]

On Thursday 24 August 2006 09:29, Glynn Clements wrote:
> Vishal Mehta wrote:
> > 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??
>
> There is a tendency to assume that all distances are in the same
> (unspecified) units, so if two elevations differ by one unit and are
> one unit apart on the ground, the slope is 45 degrees.
>
> If both coordinates and elevations are in metres, there's no problem.
> If they're both in feet, again no problem. If one is in feet and the
> other in metres, the results will be wrong. If the coordinates are in
> degrees, the results are likely to be very wrong.
>
> This problem can be solved to an extent by using the value from the
> PROJ_UNITS file to convert ground distances to metres (although that
> requires that the PROJ_UNITS file actually contains the correct
> value).

Perhaps I have missed something here, but why not project you latlong data to 
some coordinate system with x,y,z units in meters ?

-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341