Recommendation:
The fill value of a variable should be a number outside its valid data range.
Recommendation Details:
The CF _FillValue attribute is used to indicate missing or invalid data for a variable. Also, the value of the CF _FillValue attribute should match the actual fill value used for the variable in the file.
The value of the CF _FillValue attribute should be a mathematically valid number that lies outside the valid range for a variable. Please note that NaN (Not-a-Number) is neither a number nor is it mathematically valid, and, thus, should not be used as the fill value (see Recommendation 3.7 of ESDS-RFC-036).
If possible, using zero as the fill value should be avoided, because zero looks too much like a physically realistic value, and this can be confusing to the end users.
There should only be one fill value per variable. We recommend using a quality flag variable along with the CF flag_values and flag_meanings attributes to explain the various reasons for using the fill value, instead of using several special values in the variable.
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