Fill Value Should be a Number Outside Valid Range

Recommendation:

The fill value should be a number outside the valid range.

Background:

The CF _FillValue attribute is used to indicate missing or invalid data for a variable.  The fill value should be a number outside the valid range.

Using NaN (Bit-a-Number) for the fill value is inconsistent with Recommendation 3.7 of ESDS-RFC-036.

Recommendation Details:

Fill Value

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).

Also, the value of the of the _FillValue attribute should match the actual fill value used for the variable in the file.

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 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.

The missing_value attribute has been deprecated, and should not be used - the _FillValue attribute should be used instead.