[GRASS-dev] Script for converting degrees clockwise from North to GRASS convention

[Margherita Di Leo]

Moritz,

I really thank you for taking the time to discuss this.

On Wed, Jan 23, 2013 at 2:54 PM, Moritz Lennert <
mlennert at club.worldonline.be> 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
>> <mlennert at club.worldonline.be <mailto:mlennert at club.**worldonline.be<mlennert at club.worldonline.be>>>
>> 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
margherita.di-leo at jrc.ec.europa.eu

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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