r.slope.aspect

Agustin Lobo alobo at ija.csic.es
Wed Feb 10 06:32:37 EST 1999


Thanks to Markus and James Cameron for their answers.
I agree that an explicit option should be included, although
the standard should be deg. from N as we are using a GIS. In the 
meantime, just be aware of the fact that aspect as
calculated by r.slope.aspect follows the geometric 
standard instead of the geographic one: we had problems at using
r.slope.aspect for computing the incidence angle: note
that cos(10) != cos (35) (i.e., 10 deg calculated by r.slope.aspect
is 35 deg from N).

Agus


Dr. Agustin Lobo
Instituto de Ciencias de la Tierra (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona SPAIN
tel 34 93409 5410
fax 34 93411 0012
alobo at ija.csic.es
http://pangea.ija.csic.es/alobo


On Tue, 9 Feb 1999, Markus Neteler wrote:

> > Why is r.slope.aspect giving aspect as degrees from East?
> > Is not the standard degrees from N (terrain azimut)?
> > 
> > Agus
> 
> Hi Agus, hi all,
> 
> r.slope.aspect is working in a mathematical sense of angle counting.
> The "geographical" rotational direction (clockwise) and starting from north
> can be reached through a patch. It is in GRASS 4.2.1 source code
> package (src421/untested/raster/r.slope.aspect2/ tree) from McCauley.
> 
> I would suggest to include a switch in GRASS 5.0 r.slope.aspect
> to get both ways of angle counting. That should not be too much
> work.
> 
> 
>   Markus
> 
> 



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