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[cf-satellite] Proposal for handling multiple "missing_value" values

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In the ESIP Federation CF-Satellite session yesterday (July 14, 2011), I suggested that it would be good to have a standard scheme for handling multiple marker values in satellite data that indicate missing data. There is confusion in the current documentation (between netCDF and CF) about the proper use of the missing_value attribute, in particular about whether or not it is acceptable for it to be a vector attribute. I think there is good reason to allow it to be a vector attribute, and that allowing the attribute to be a vector quantity is useful beyond the satellite / remote sensing arena. The problem that arises with having a vector missing_value attribute is the assignment of meanings to the different values.
One way to deal with this is to have a missing_value_meanings attribute. The missing_value_meanings attribute would be formed in the same way as the flag_meanings attribute. There would be one string of non-whitespace characters per element of the missing_value attribute, separated by spaces. Each string would be a human-readable phrase (delineating the words of the phrase using underscores or camel-casing) that explained the meaning associated with the corresponding missing_value element. Here's an example fragment of cdl. 

   float foo(points);

       foo:missing_value = -999.1, -999.2;
       foo:missing_value_meanings = "InputsOutOfRange
       SolutionDidNotConverge";

Grace and peace,

Jim Biard

--
Jim Biard

Government Contractor, STG Inc.
Remote Sensing and Applications Division (RSAD)
National Climatic Data Center
151 Patton Ave.
Asheville, NC 28801-5001

jim.biard@xxxxxxxx
828-271-4900
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