On 23/01/13 13:59, Margherita Di Leo wrote:
Hi Moritz,
thank you for your reply,
On Wed, Jan 23, 2013 at 12:21 PM, Moritz Lennert
<[email protected] <mailto:[email protected]>> wrote:
If we call WIND the direction map which uses the convention degrees
clockwise from North, and GRASS the output map, the rule that I
applied is:
IF 0 <= WIND < 270 ; GRASS = 270 - WIND
IF WIND = 270 ; GRASS = 360
IF 270 < WIND < 360 ; GRASS 630 - 0
IF WIND = null ; GRASS = 0
NULL otherwise (this latter I have added to check if there is
any error.
This is not correct.
WIND = 0 means North.
Actually this was tricky also for me, but:
Wind direction is measured in:
*
degrees clockwise from uphill (direction the wind is pushing the fire)
*
degrees clockwise from North (direction the wind is blowing from)
If the wind is blowing FROM north, it means that the arrow is TOWARDS
south.. Am I wrong?
I guess not, but this is a very specific application of angle direction.
I think that if you want to create a generic module (r.convert.aspect or
r.convert.angle) for transforming angles, then North should mean North.
If for a specific application North means a direction a force is coming
from, then the use should take that into account. In your case the
original map gives the angle the wind is coming _from_, then the output
should (IMHO) do the same.
Moritz
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