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_meanings attributes to explain the various reasons for using the fill value, instead of using several special values in the variable.
Awaiting ESO Approval
This recommendation has been finalized by DIWG but has not yet received final ESO approval.