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

[Glynn Clements]

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).

-- 
Glynn Clements <glynn at gclements.plus.com>