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

[Helena Mitasova]

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
>>
>> Bob
>>
>>
>> Bob Moskovitz
>> Research Analyst I
>> Seismic Hazard Zonation Project
>> California Geological Survey
>> http://www.conservation.ca.gov/cgs/shzp
>>
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