Moritz, I really thank you for taking the time to discuss this.
On Wed, Jan 23, 2013 at 2:54 PM, Moritz Lennert < [email protected]> wrote: > 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:mlennert@club.**worldonline.be<[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. > This would be of no use for me (or the user) if it doesn't match to the GRASS convention. So what I suppose I should do is to explain in the documentation that the input should be measured from SOUTH (0=SOUTH) clockwise. Thanks! Madi > > Moritz > -- Margherita DI LEO Postdoctoral Researcher European Commission - DG JRC Institute for Environment and Sustainability (IES). Unit H03 – FRC Via Fermi, 2749 I-21027 Ispra (VA) - Italy - TP 261 Tel. +39 0332 78 3600 [email protected] Disclaimer: The views expressed are purely those of the writer and may not in any circumstance be regarded as stating an official position of the European Commission.
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