Hi Jonathan, I'm wondering about this statement:
> I don't think it's misleading. This is a consistent rule in CF standard > names: quantities with different canonical units must have different standard > names. Isn't the use of "cell_methods" with a value of "variance" an example where we don't require a different standard_name even though the units are different? I would have said that standard names need to match and units need to be conformable in order for quantities to be comparable. Brian On Thu, Jun 17, 2010 at 09:03:47AM +0100, Jonathan Gregory wrote: > Dear Benno > > I think our arguments are all reasonable and clearly stated. The conclusion > is not clear. Other points of view would be useful. > > > 1) Fourier transform is a change of basis, not physical variable -- it > > is invertible as a matter of fact, as long as you keep the real and > > imaginary parts. Because it is a change of basis, it is much more > > analagous to a projection transformation than a transformation of the > > physical variable. > > > > There is a change of units, you are right, but that is misleading if > > it leads you to think that more than a change of basis has occured. > > I don't think it's misleading. This is a consistent rule in CF standard > names: quantities with different canonical units must have different standard > names. Actually that's not because the units are somehow of magical > significance, but different units do imply some significant difference in > definition. A quantity and its Fourier transform are clearly not comparable > in the sense that you could, for instance, try to calculate their difference. > If the standard name is the same there is no distinction in CF metadata > between them except inspection of their coordinate dimensions, which is not > where a generic application would expect to look. It might search a file for > a variable of air_pressure and find a Fourier transform of air pressure by > looking at the standard names, and that's not what it would want. > > I take your point that this is the same kind of situation as a change of > spatial grid of a given quantity e.g. by interpolation, where the only > distinction is that the spatial coordinate variables have changed. Still I > think that is a smaller change than a Fourier transform. After a grid-mapping > operation, you do still have horizontal spatial coordinate variables. After > a Fourier transform, one of the spatiotemporal coordinate variables has > disappeared altogether and been replaced with a different coordinate. I would > guess that the average scientific user of the data would be more likely to > regard air_pressure on two different horizontal grids as the "same geophysical > quantity", but air_pressure and its Fourier transform wrt time as "different > geophysical quantities". That may sound arbitrary and hence the change of > units is a useful practical guide to deciding. > > > 2) Software that manipulates complex numbers, (including FFT), needs > > both the real and imaginary parts -- putting them in separate > > variables is a highly artificial reconstruction of the data which has > > to be undone in order to proceed with any manipulation that treats it > > as complex numbers. Also, the software needs to detect the components > > of the complex numbers -- you are making it much more difficult. The > > point of metadata is to make the necessary information available > > clearly and unambiguously -- having a dimension clearly labelled as > > corresponding to real and imaginary is much closer to the information > > needed to write software to correctly handle complex data. > > Again, I understand that point. It could be a bit less convenient, but surely > it's not *that* bad, is it? It's no harder to look for two different standard > names than two different values of a coordinate variable. The labelling of > real > and imaginary in the standard name would be just as clear and unambiguous as > putting it in a coordinate variable - possibly more obvious, in fact. > > My argument for doing it this way is that it's simpler i.e. it just needs the > standard name, no more machinery, and because it's what we do in other cases > where there are a small number of components - usually two (xy) or three > (xyz). > > > 3) CF should handle spectral harmonics as well. > Yes, when it is requested to do so. I'd probably agree with you that in that > case we would have a coordinate dimension, because it would be multivalued and > the number and identity of the components would depend on the spectral > resolution. > > Best wishes > > Jonathan > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
