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

[Moskovitz, Bob]

Thank you for the tip.  Btw, I was made aware of this "problem" when a coworker took our landslide hazard workflow and tried to apply it to grass.  Everything look great, but when we took another look at the various layers, we were shocked to see how aspect was defined.  I know...we should have read the manuals, but everybody knows how aspect is defined.  Anyways, we can live with another step in our process, but it would be great if there was a document that shows the differences between GIS systems (such as ESRI, MapInfo, etc), so that we can keep surprises at a minimum. :-)

Bob Moskovitz
Research Analyst I
Seismic Hazard Evaluation Project
California Geological Survey
http://gmw.consrv.ca.gov/shmp

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> -----Original Message-----
> From: Hamish [mailto:hamish_b at yahoo.com]
> Sent: Tuesday, January 20, 2009 12:50 PM
> To: Grass-User (E-mail); Moskovitz, Bob
> Subject: Re: [GRASS-user] Aspect direction in r.slope.aspect
> 
> 
> Bob Moskovitz wrote:
> > 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.
> 
> 
> It is just measuring theta positive from the x-axis, in the 
> mathematical
> sense.
> 
> It stays like that for historical reasons, probably been that 
> way for 20+
> years. (aka we can only guess at the original logic)
> 
> Convert between that and the compass (CW from north) convention with
> "90 - angle". Most modules which deal with aspect maps should already
> have an option to read the different type.
> 
> 
> Hamish
> 
> 
> 
>       
> 
>