Analyzing landsurface using FFT

Dear members,

I am trying to analyze the sinusoidal surface of a lava-flow for dominant wavelengths/frequencies using the FFT command. I made traverses of the lava-flow to have 2 dimensional elevation data which is stored in a vector. I read a lot about this function but don't understand yet how to use it for my purpose. All the examples on the web define a sampling frequency in the beginning but in this particular example I just have a vector with a datapoint every meter. So as I don't generate the sinusoidal wave myself I don't know all the parameters like sampling frequency etc. It is just an irregular wave.

Can somebody help me out with that? Is the fft a good approach for this project? Should I just define any random sampling frequency?

Thanks a lot, I am kind of new to the fft-function. yves

Answer 1

[Jeremy]

If you have a data point every meter, then that is your "sampling rate", 1 sample/meter. Your fft will result in frequency bins that range from 0 cycles per meter to .5 cycles per meter. If you do not have data that is exactly every meter than you may want to interpolate to every meter.

Since you have elevations you will probably want to normalize your data by subtracting out the mean or min value, other wise you will have a very large values in the first bin.

[Jeremy]

This is an interesting use of a Fourier transform, I would like to see your results if possible.