I am working with a dissolved oxygen dataset.  continuous readings are taken
at 15 minute intervals and we have been recording these data at 12 stations
along the savannah river for two years now.  The longest set of readings
that are continuous without interuption is 53 days.  I would like to look at
the power spectral density at each of these sites (most likely one day will
be the overridding trend due to photosynthesis and respiration).  I would
also like to look at the cross spectrums between to contiguous site to look
for advective transport from the upstream sections.I have used spectrum to
get a periodogram.  This I don't believe is the power spectral density.  If
it is I am unsure how to determine the # in the  span argument to smooth the
periodogram.  I imagine that I am overlooking something- I appreciate all of
your help.

stephen

-- 
Let's not spend our time and resources thinking about things that are so
little or so large that all they really do for us is puff us up and make us
feel like gods.  We are mammals, and have not exhausted the annoying little
problems of being mammals.

-K. Mullis

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