Dear list,
I apologies first for my English, hope you will understand well my question.
I am working on 1/2 hour piezometric data, time unit is second. They
present daily oscillation when using the spectrum() function. What I am
really interested in, is to find the amplitude corresponding to this
oscillation.
I work with a college using Matlab, and although we apply the same
methodology, our results differs : we find the same frequencies, but not
the same amplitudes.
I put the code I use here to understand if the difference comes from the
way that fft function are encoded int both software or from my own
misunderstanding of the function.
I am importing data as zoo, and verifying that they are regular spaced,
and after, I simply use the non smoothed spectrum() function :
SP<-spectrum(as.ts(Piezo))
To recuperate the amplitude I calculate the square root of the density,
divided by the number of observations at the the frequency I am
interested in (1/86400=frequency of one cycle per day) :
sqrt(SP$spec[which(SP$freq==1/86400)])/length(Piezo)
Does anyone of you use both Matlab and R and have already find
differences in results ? Or does my trouble comes from my own methodology ?
Thank you very much in advance for your feedback,
--
---------------------------------------------------------------------------
Marie Guillot
PhD Student
INRA - UR1263
Centre de Recherche de Bordeaux
Unité de Recherche EPHYSE
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
