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Re: Using 1D FFT to decompose the provided hurricane data in terms of wavenumbers. [message #90494 is a reply to message #90487]

Thu, 05 March 2015 07:32

Burch
Messages: 28
Registered: December 2013

Junior Member

On Wednesday, March 4, 2015 at 5:11:01 PM UTC-6, twie...@fiu.edu wrote:
> I have 240x240 array of hurricane wind speed.
>
> 1) I need to covert to polar coordinates and plot the speed on a contour map with a radius of [-108,108], however what I have doesn't appear to be correct.

> Theta=atan(Y/X)*2*!PI

Calculating theta in this way will give you incorrect results. For instance, notice that

IDL> y = 1.0/2.0
IDL> x = -1.0/2.0
IDL> print, atan(y/x)
-0.785398

and

IDL> y = -1.0/2.0
IDL> x = 1.0/2.0
IDL> print, atan(y/x)
-0.785398

give the same result even though the locations are in different quadrants! You should use the two argument form of atan()

Theta = atan(y, x)

This will give you results ranging from -pi to pi. To change to 0 to 2 pi do

Theta = (theta + 2.0*!pi) mod (2.0*!pi)

-Jeff

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[Message index]

		Using 1D FFT to decompose the provided hurricane data in terms of wavenumbers. By: twiel002 on Wed, 04 March 2015 15:10
		Re: Using 1D FFT to decompose the provided hurricane data in terms of wavenumbers. By: Burch on Thu, 05 March 2015 07:32
		Re: Using 1D FFT to decompose the provided hurricane data in terms of wavenumbers. By: Burch on Thu, 05 March 2015 07:41

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