issue_comments: 895336362

It's in the standard (partly?): https://www.unidata.ucar.edu/software/netcdf/documentation/4.7.4-pre/file_format_specifications.html#atts_spec // Default fill values for each type, may be // overridden by variable attribute named // '_FillValue'. See "Note on fill values", // below. FILL_CHAR = \x00 // null byte FILL_BYTE = \x81 // (signed char) -127 FILL_SHORT = \x80 \x01 // (short) -32767 FILL_INT = \x80 \x00 \x00 \x01 // (int) -2147483647 FILL_FLOAT = \x7C \xF0 \x00 \x00 // (float) 9.9692099683868690e+36 FILL_DOUBLE = \x47 \x9E \x00 \x00 \x00 \x00 \x00 \x00 //(double)9.9692099683868690e+36

and

Note on fill values: Because data variables may be created before their values are written, and because values need not be written sequentially in a netCDF file, default “fill values” are defined for each type, for initializing data values before they are explicitly written. This makes it possible to detect reading values that were never written. The variable attribute “_FillValue”, if present, overrides the default fill value for a variable. If _FillValue is defined then it should be scalar and of the same type as the variable.

Fill values are not required, however, because netCDF libraries have traditionally supported a “no fill” mode when writing, omitting the initialization of variable values with fill values. This makes the creation of large files faster, but also eliminates the possibility of detecting the inadvertent reading of values that haven't been written

EDIT: I remember reading some text about how the default _FillValues are "close to the largest or smallest number representable by a datatype", but I cannot find it now.