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Russ,I'm confused about a few things. If you don't mind spending a bit of time clarifying, I'd appreciate it.
Here are my confused questions.
* Isn't the _FillValue attribute the one that maps to '_'? (See the
discussion of _FillValue and missing_value here
<http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/Attribute-Conventions.html>.)
* Is the flag_values attribute considered applicable in the case of
floating point variables?
* In what sense is this suggestion making a change to the
interpretation of the missing_value attribute?
Grace and peace,
Jim
On 7/15/2011 3:09 PM, Russ Rew wrote:
Hi Jim,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";I agree that it would be very useful to have an attribute for multiple special values for satellite data as well as other kinds of data. IIRC, you gave an example of 10 different kinds of missing values for an NPP sensor. I'm a little concerned about breaking backward compatibility by extending the "missing_value" attribute for this purpose, because there may be software that depends on a one-to-one mapping between a missing_value for a variable and some alternate representation for it, such as the '_' notation ncdump already uses. The fact that the ncdump and ncgen utilities are inverses of each other, converting netCDF data to CDL and back to netCDF without changing the data, requires a single value for the missing value to avoid loss of information when converting from '_' back to the corresponding value. I suggest just using the existing CF flag conventions for this purpose instead, using the variable attributes "flag_values", and "flag_meanings" for any number of special values for a variable. If one of those values is a missing value, the associated flag_meaning could just be "missing_value", but flag_meanings can also specify an arbitrary number of other nuanced kinds of missing values or various other kinds of special values. http://cfconventions.org/documents/cf-conventions/1.5/cf-conventions.html#flags --Russ _____________________________________________________________________ Russ Rew UCAR Unidata Program russ@xxxxxxxxxxxxxxxx http://www.unidata.ucar.edu
-- 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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