Ken Knapp wrote:
I think the need for variable scale/offsets for each band is a
different issue as it would not be specific to satellite data.
True, but necessary in many cases of satellite data.
Tom
-Ken
John Caron wrote:
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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