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

[Nikos Alexandris]

On Tue, 2009-01-20 at 12:50 -0800, Hamish wrote:
> 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

This was a small discussion I had today. Supporting an undergraduate
student to work on his thesis, I produced an aspect map for him some
weeks ago. And back then I wrote it *clearly*, in a mail of min towards
him, the "Aspect map values start from EAST and increase
counterclockwise".

Now, he was unsure, so he asked some other guy and he told him... well
you know the rest of the story. Then I pointed to r.slope.aspect manual
and told him to read it. Of course I could re-calculate the values, but
sometimes I prefer to leave thins as they are produced by the module. So
I don't need to take notes about that.

Nikos