Fwd: [GRASS-user] r.series and Fourier
paulomarcondes at gmail.com
Fri Jun 13 08:40:18 EDT 2008
This should have gone to the list...
2008/6/12 Glynn Clements <glynn at gclements.plus.com>:
> Daniel Victoria wrote:
> An FFT would produce a series (or two series - real/imag or arg/abs)
> of outputs. If you just want a single frequency component, you would
> need to pass the frequency (or period) as an argument, and r.series
> doesn't support that yet.
> A secondary issue is that GRASS' FFT function is optional, and is only
> available if built with the --with-fftw option.
> So, it's mostly a question of whether to add Fourier support to the
> existing r.series module, or to add a separate r.series.fft module
> (which could be based upon r.series).
> For a separate module, most of the effort required is in
> specification; if it's known exactly how the module is meant to
> behave, implementation would be around an hour's work.
I see Glynn's points as quite reasonable.
Besides doing fourier over a time series of maps, I think r.series.fft
or r.fourier would have to do spectral decomposition: generate several
maps, one for each specified frequency range. maybe inputing max and
min freq + step size or simply number of spectral slices to take.
Well, I'm talking spatial frequencies here, the kind that goes [L]^-1
I am not sure if these functionalities fit in the same module, but,
since we are talking fourier, I chimed in =]
Paulo Marcondes = PU1/PU2PIX
-22.915 -42.224 = GG86jc
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