[GRASS-dev] Re: [GRASS-user] Aspect direction in r.slope.aspect

[Dylan Beaudette]

On Tuesday 20 January 2009, Helena Mitasova wrote:
> On Jan 20, 2009, at 3:16 PM, Markus Neteler wrote:
> > Helena,
> >
> > I guess this is a question for you. If you post from
> > hmitaso at unity.ncsu.edu it
> > reaches grass-user directly.
> >
> > Markus
> >
> > On Tue, Jan 20, 2009 at 8:39 PM, Moskovitz, Bob
> >
> > <Bob.Moskovitz at conservation.ca.gov> wrote:
> >> Hello Grass Users,
> >>
> >> I was just wondering why aspect is measured from the east and
> >> increased
> >> counterclockwise instead of the north and clockwise?  It seems to run
> >> counter to what is done in other GISs.
>
> at the time when it was implemented there was no other major raster
> GIS (no Spatial Analyst, only
> some research software and each used different rule, based on the
> background
> of the developer), so it was done to conform how the angles are
> measured in math (east is your +x
> and the values increase counterclockw).
> I vaguely remember discussions on whether to change it to conform with
> the "aspect as azimuth" concept with 0 pointing towards North but
> there were already too many
> people used to the way it was originally implemented that it was
> safer to keep it.
> (we had to make the decision for aspect in v.surf.rst as well and we
> decided
> to keep the math convention to make it consistent with r.slope.aspect).
>
> GRASS7 provides opportunity to change it and make sure that all
> modules that compute and use
> aspect or flow direction conform to the same rule. But it would be a
> major undertaking that could
> break a lot of scripts - I am not sure it is worth it and whether
> anybody would actually be
> willing to do it - it really depends on what your background is.
> So if there are people who think this is really important and there
> is some official standard that
> says what it should be and there are some volunteers to actually work
> on it, please post
> to the grass-dev list,
>
> Helena

Good topic for discussion. I think that GRASS7 should stick with the current 
conventions to retain backwards compatibility. Since the reference of the 
aspect calculation is clearly spelled out in the manual there should be no 
problem- as long as people read the manual. Perhaps a careful check of all of 
the manual pages associated with directional calculation should be checked to 
make sure that there is a note.

Cheers,

Dylan


> >> Bob
> >>
> >>
> >> Bob Moskovitz
> >> Research Analyst I
> >> Seismic Hazard Zonation Project
> >> California Geological Survey
> >> http://www.conservation.ca.gov/cgs/shzp
> >>
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-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341