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

[Glynn Clements]

Glynn Clements 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.

Ignore that last part; r.slope.aspect actually ignores that value, and
instead uses G_distance(), which has a special case for lat/lon.

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