# [gradsusr] Plot vector - geographic X trigonometric system

Jeffrey Duda jdduda at iastate.edu
Mon Sep 27 16:16:15 EDT 2010

```Oh ok.  I get it now.  yeah, I've run into this problem myself.  Try using

u = wind_magnitude * cos(450 - degrees_in_your_data)
v = wind_magnitude * sin(450 - degrees_in_your_data)

Jeff

On Mon, Sep 27, 2010 at 2:26 PM, Gabriel - ud Carvalho <gabriel.ud at gmail.com
> wrote:

> I think that I was not clear,
> I can´t do this suggestion because the 'sin' function is oriented to the
> trigonometric system, which isn´t the system in which my data is oriented.
> For instance, in my data, a 270 degrees is equivalent to 180 degrees in the
> trigonometric system.
>
> I hope I am more clear now, even thoug the roughness of my english...:)
>
> Gabriel  Carvalho
>
> 2010/9/27 Jeffrey Duda <jdduda at iastate.edu>
>
>> Gabriel,
>> As long as your data contains wind direction oriented such that 0 deg in
>> your data refers to a wind blowing to the north (or from the south), then
>> your components can be broken down as:
>>
>> u = wind_magnitude * sin(direction)
>> v = wind_magnitude * cos(direction)
>>
>> Of course, when actually putting these computations into Grads you will
>> want to change "direction" from degrees to radians.
>>
>> Jeff Duda
>>
>>  On Mon, Sep 27, 2010 at 12:34 PM, Gabriel - ud Carvalho <
>> gabriel.ud at gmail.com> wrote:
>>
>>>  Hello you all!
>>> Here is the deal:
>>> In my data, the wind direction is in degrees, with a geographic
>>> representation (wich means that 0 is at the north and goes clockwise),
>>> the problem is that to display this data with 'gxout vector', I have to
>>> decompose it in 'u' and 'v'. In order to do this, I have to use the
>>> trigonometric functions 'sin' and 'cosin'. The problem is here, these
>>> functions assume that the input data is with a trigonometric representation
>>> (wich means that 0 degrees is eastside and goes anti-clockwise),butthey
>>> aren´t, so, it got all messed up.
>>>
>>> I don´t know what to do,but I believe that this is a common problem,
>>> I hope someone will know the answer...
>>>
>>> Sorry about my poor english,
>>>
>>> My best regards,
>>> Gabriel Carvalho
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>>
>>>
>>
>>
>> --
>> Jeff Duda
>> Iowa State University
>> 3134 Agronomy Hall
>> www.meteor.iastate.edu/~jdduda <http://www.meteor.iastate.edu/%7Ejdduda>
>>
>> _______________________________________________
>>
>>
>
> _______________________________________________
>
>

--
Jeff Duda
Iowa State University