Re: [cf-satellite] Proposal for band dimension and coordinate variable

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On 7/21/2011 10:15 AM, Ken Knapp wrote:
CF-SAT Listers-
Based on the various ways that bands/channels can be differentiated, I lean toward the first solution that Tom described (shown below).

On 7/20/2011 3:37 PM, Tom Whittaker wrote:
One simply used a name like
"band" solely as a dimension and defined a standard_name for this so
that code could recognize it as such.  Several other variables were
then used to define wavelength, etc., again using standard_names to
alert applications as to what they were.

The problem with the other is that there so many ways to differentiate channels. Rudi brought up polarization. Other issues include: - You'd also need frequency as a std variable (since microwave channels are generally titled that way). - Channels with same frequency but variable widths [where the center frequency is the same, but the range od sensitivity changes, like with SSMIS (http://en.wikipedia.org/wiki/SSMIS) and AMSU which is even more complex (http://en.wikipedia.org/wiki/Advanced_Microwave_Sounding_Unit) -Similar frequency but with differing optical path cell pressure (something sits in the optical path on the satellite ... a la SSU) http://www2.ncdc.noaa.gov/docs/podug/html/c4/sec4-2.htm


So I would recommend not creating coordinate axis types because there are so many ways to differentiate "bands".

Thoughts?
-Ken


If one adds a dimension, say, "band", to a variable, an application might want two different things:

1) some "user readable" coordinates that allow a user to select the band that they want. this comes up in GUIs like the IDV, and in web services like WMS. We dont really care what the semantics are, as long as the user understands what they are selecting.

2) more sophisticated apps might want to do various calculations, based on understanding what the value of wave length, wave number, polarization, etc.

For the first case, we need some (any) standard coordinate axis for the band dimension. I think probably wavenumber and wavelength would suffice, ie all data could use one or the other.

For the second we can use as many variables as are needed / wanted. One can make them into auxilary coordinates simply by adding the variables' names to the data "coordinates" attributes. This doesnt help generic applications much unless those variables are given a standard name in order to describe their meaning. So the cf-satellite group could come up with a set of standard names for this purpose, eg "polarization", "wavelength_width", etc.

So i would say the proposal could define new axis types "wavenumber" and "wavelength", and require all conforming files to use one or the other as a coordinate (regular or auxiliary) for the band dimension (the actual dimension name can be anything). Define other variables as your community needs, clarify the semantics using a standard_name, and recommend that these are used as needed as auxiliary coordinates.

john






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