CODEWITHSUNDEEP
matlabffttidalcycles
ConnorG
ConnorG

Reputation: 71

Resolved Frequencies of a Fourier Transform from a Time series

I have a time series of some tidal data which is given as the following: Original time series of Tidal Elevation Data

If we take the FFT to determine the specific harmonics present in the data we see the the following: FFT of Tidal Elevation data

This FFT was computing using the matlab FFT function from the method presented here(https://www.mathworks.com/matlabcentral/answers/435508-how-can-i-get-proper-length-of-fft)

Fn = Fs/2;   % Nyquist Frequency
N = length(z); %Compute length of Elevation z
NFFT = 2^nextpow2(N); 
FTx = fft(z, NFFT) / N;
Fv = linspace(0, 1, fix(N/2)+1)*Fn*60*60*24;% Frequency Vector from seconds to days
Iv = 1:numel(Fv);    
figure(5)
semilogy(Fv, abs(FTx(Iv))*2)
grid on;

The issue is I know there are harmonics of the tide which occur on scales greater than 14 days however the basic FFT seems to choose the frequency limits it returns to 12 days despite my time series having many months of data, How can I force the FFT to incorporate those frequencies?

Upvotes: 0

Views: 158

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