Fourier Transform Help

Question

I'm trying to do a Fourier Transform of the following Data, but I'm completely stuck.

Here's what I've done so far:

data = Drop[data, 1];
dat = data /. {x_, y_, z_, w_} -> {x + y/12., 10 z};
ListLinePlot[Abs[Fourier[dat]], PlotRange -> {Automatic, {0, 40}}]

I don't understand why it keeps displaying this weird plot

Anyone have any clue what I'm doing wrong? I'm just learning this Fourier thing so I'm real lost.

Answer 1

Fourier does not accept a list of {time, sample} pairs - if you give it a 2D list it will do a 2D transform. Therefore you need to use Fourier on just the list of samples. To get the correct frequency scale on the plot you can use DataRange.

data = Import["http://solarscience.msfc.nasa.gov/greenwch/spot_num.txt", "Table"];

dat = Rest[data] /. {x_, y_, z_, _} :> {x + y/12., 10 z};

n = Length[dat];
t = dat[[2, 1]] - dat[[1, 1]];
fourierdata = Abs @ Fourier[dat[[All, 2]]];

ListLinePlot[fourierdata[[;; 1 + n/2]],
 DataRange -> {0, 1/(2 t)},
 PlotRange -> {{0, 0.5}, {0, 10000}}]