CODEWITHSUNDEEP
pythondegreesradians
chantal dem
chantal dem

Reputation: 75

Python Arcsin Arccos radian and degree

I am working on wind power and speed

u and vare zonal and meridional wind. (I have the values of these 2 vectors)

The wind speed is calculated by V = np.sqrt(u2*v2)

Wind direction is given by α between 0 and 360 degree

I know this relation holds - u / V = sin( abs(α)) and - v / V = cos( abs(α))

In python I am using np.arccos and np.arcsin to try to find α between 0 and 360 with the 2 equation above. For the first one, it returns the radian so I convert with np.rad2deg(...) but it gives me a value between 0 and 180 degree for the second one, I also try to convert but it returns me a valus between 0 and 90.

Anyone knows how to code it? I am lost :(

Upvotes: 0

Views: 1386

Answers (1)

Serge Ballesta
Serge Ballesta

Reputation: 149075

The underlying problem is mathematics: cos(-x) == cos(x), so the function acos has only values in the [0,pi] interval. And for equivalent reasons asin has values in [-pi/2,pi/2] one.

But trigonometric library designers know about that, and provide a special function (atan2) which uses both coordinates (and not a ratio) to give a value in the [-pi, pi] interval.

That being said, be careful when processing wind values. A 360 wind is a wind coming from the North, and 90 is a wind coming from the East. Which is not the way mathematicians count angles...

Upvotes: 1

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