The NUG conventions ( NUG section 8.1 NUG section 8.1 ) provide the _FillValue, missing_value, valid_min, valid_max, and valid_range attributes to indicate missing data.

The NUG conventions for missing data changed significantly between version 2.3 and version 2.4. Since version 2.4 the NUG defines missing data as all values outside of the valid_range, and specifies how the valid_range should be defined from the _FillValue (which has library specified default values) if it hasn't been explicitly specified. If only one missing value is needed for a variable then we recommend strongly that this value be specified using the _FillValue attribute. Doing this guarantees that the missing value will be recognized by generic applications that follow either the before or after version 2.4 conventions.

The scalar attribute with the name _FillValue and of the same type as its variable is recognized by the netCDF library as the value used to pre-fill disk space allocated to the variable. This value is considered to be a special value that indicates undefined or missing data, and is returned when reading values that were not written. The _FillValue should be outside the range specified by valid_range (if used) for a variable. The netCDF library defines a default fill value for each data type ( NUG section 7.16 NUG section 7.16 ).

The missing_value attribute is considered deprecated by the NUG and we do not recommend its use. However for backwards compatibility with COARDS this standard continues to recognize the use of the missing_value attribute to indicate undefined or missing data.

The missing values of a variable with scale_factor and/or add_offset attributes (see section Section 8.1, “Packed Data”) are interpreted relative to the variable's external values , i.e., the values stored in the netCDF file. (a.k.a. the packed values, the raw values, the values stored in the netCDF file), not the values that result after the scale and offset are applied. Applications that process variables that have attributes to indicate both a transformation (via a scale and/or offset) and missing values should first check that a data value is valid, and then apply the transformation. Note that values that are identified as missing should not be transformed. Since the missing value is outside the valid range it is possible that applying a transformation to it could result in an invalid operation. For example, the default _FillValue is very close to the maximum representable value of IEEE single precision floats, and multiplying it by 100 produces an "Infinity" (using single precision arithmetic).

This convention defines a two-element vector attribute actual_range for variables containing numeric data. If the variable is packed using the scale_factor and add_offset attributes (see section 8.1), the elements of the actual_range should have the type intended for the unpacked data. The elements of actual_range must be exactly equal to the minimum and the maximum data values which occur in the variable (when unpacked if packing is used), and both must be within the valid_range if specified. If the data is all missing or invalid, the actual_range attribute cannot be used.