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On 5/20/2010 12:42 PM, Tom Rink wrote:
John Caron wrote:Hi Tom, thanks for taking up this discussion.From my POV, theres no problem with a "band" or "wavelength" coordinate variable. Coordinate variables are very general, I think of geospatial coordinate variables as a special case. So I would strongly advocate a "band" coordinate variable, where the coordinate values would specify somethinkg useful, eg the wavelngth or frequency, rather than just a dimension.Theres no problem with multiple bands in a file, since they would use different dimensions, and the coordinates atttribute can disambiguate if needed.We would need to modify the CDM library to deal with this, but we would definitely do this once we had a clear CF spec.A vector scale/offset for each band is a more difficult problem, and i think there have been previous suggestions on this. Its really a concept that could be used in many situations, so it might be good to think of the general case. OTOH, with compression in netcdf-4, there is some opinion that these kinds of tricks are less useful than when we had only netcdf-3.A variable probably shouldnt have different units, however. That would be a good argument for a seperate variable. Would this cause a variable explosion?I think as long as 2 or more 'band' dimensions could be defined, a single unit per variable will work. I'm wondering though if variables for scale/ offset/unit might not be the more general approach. Since scaleFactor, addOffset, Unit are standard names anyway, couldn't they just be the standard name of said variables? This would address the netcdf-4 compression you mentioned above.
Hi Tom:(For some reason your message didnt go directly to the maillist, not sure why)
If seperate scale/offset for each band are needed, im sure we can work out a mechanism for that. A seperate unit would be more problematic and I would advise to avoid if possible. The reason is that a single name/unit/description is the essence of what a variable is. The scale/offset is really more of a storage option that should be transparent to higher software levels (and is transparent in the netcdf-4 format).
It would be helpful if potential satellite data writers looked at their data under those assumptions (seperate unit == seperate variable; do you need distinct scale /offsets per band) to see how that would play out with their particular datasets.
John
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