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

[Glynn Clements]

Vishal Mehta wrote:

> > 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).
> 
> I did'nt fully understand this comment Glynn. Could you please elaborate
> this idea?
> FYI,
> GRASS 6.0.1 (cru3):~/CRU > g.proj -w
> GEOGCS["wgs84",
>     DATUM["WGS_1984",
>         SPHEROID["wgs84",6378137,298.257223563],
>         TOWGS84[0.000,0.000,0.000]],
>     PRIMEM["Greenwich",0],
>     UNIT["degree",0.0174532925199433]]
> 
> And my Proj_units file in the PERMANENT mapset is:

> unit: degree
> units: degrees
> meters: 1.0

The "meters" value in the PROJ_UNITS file is supposed to be a scale
factor which converts map units to metres (i.e. 1.0 for metres, 0.3048
for feet). For degrees, it should be roughly 111300 * cos(latitude).

r.slope.aspect uses that value to convert map units to metres; if it's
set correctly (and the zfactor= option is set correctly if your
elevations aren't in metres), your slope map should be correct. There
might still be problems with r.sun itself, though.

> I'm not sure what the last line of g.proj -w means, what does unit there
> refer to, as opposed to the proj_units 'meters' field?

0.0174532925199433 is pi/180, i.e. it represents the size of the
(angular) units in radians.

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