Hi Jonathan
I wasn't thinking about amp and phase, primarily because I had a specific
use case in mind (as you appear to do, so that's both of us :-)
I concede that you can make an argument that in *some* cases you might
only use one component from amp and phase, but I can't think of any
mainstream applications where you would only use one of the real and
imaginary components, and I can think of plenty of use cases for
decomponsing that way (I am sure someone could think of an excption usin
one of real or imaginary, but it's not going to dominate over the other
use cases).
So, I don't think we should shunt the solution for this into one use
case of FTs, which I concede could be handled as you describe where it
the only one. But, I still wouldn't like it. It feels wrong (but then
I'd rather like CF to evolve to supporting vector concepts better
anyway, and I lost that argument years ago).
Bryan
p.s. most of the spectral data I wrote back in the days when I wrote
data of my own, used real+imaginary, even when the analysis code wanted
amplitude and/or phase. Nag algorithms for example (used to?) do that
... so it's the natural storage route.
On Thursday 08 Jul 2010 16:56:17 Jonathan Gregory wrote:
> Dear Bryan and Benno
>
> > I do *not* think it is appropriate to split the real and imaginary
> > parts into two variables, since one almost never uses just one or
> > the other, so it's a rather different case from splitting vector
> > components.
>
> I disagree still. Fourier transforms don't have to expressed as real
> and imaginary components, as you know. It can equivalently be done
> as amplitude and phase, and in that case it is much more likely you
> might use one without the other. Of course you need both to reverse
> the transformation, but for other purposes you might be interested
> in just one of them.
>
> Fourier transform components as amplitude and phase have different
> units and are, I would say, different kinds of geophysical quantity.
> I don't think it would be right to give them a common standard name
> and put them in the same data variable. Instead, consistent with
> what I suggested earlier, I think we should put them in separate
> data variables with standard names such as
> amplitude_of_fourier_transform_wrt_time_of_sea_surface_temperature
> phase_of_fourier_transform_wrt_time_of_sea_surface_temperature These
> variables have a coordinate of frequency. If it was a FT wrt
> longitude, they would have standard names with wrt_longitude instead
> of wrt_time, and a coordinate of, say, reciprocal_of_longitude.
>
> Best wishes
>
> Jonathan
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>
--
Bryan Lawrence
Director of Environmental Archival and Associated Research
(NCAS/British Atmospheric Data Centre and NCEO/NERC NEODC)
STFC, Rutherford Appleton Laboratory
Phone +44 1235 445012; Fax ... 5848;
Web: home.badc.rl.ac.uk/lawrence
Received on Thu Jul 08 2010 - 14:31:27 BST
