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Summary: [ferret_users] missing value not detected in double-precision ferret

Dear ferret users,

Our climate model output has many variables of the type "float" = single precision. Reading this data into double-precision ferret caused "-1.E34" not to be recognized by ferret anymore as missing/bad value. "-1.E34" had been written as data into the files where data was missing/bad, but had not be explicitly defined among the variable attributes in the netCDF header.


1) "missing_value" or "_FillValue" has to be defined explicitly as a variable attribute for single-precision variables.

2) The safest way to do this is to specify the "_FillValue", best if specified explicitly with a type "f" as "-1.E+34f", even
though "_FillValue" also works without specifying the type. If
"missing_value" is preferred, it HAS to be specified with a type as
given above.

float press(time) ;
                press:long_name = "pressure" ;
                press:_FillValue = -1.E+34f ;
                press:missing_value = -1.E+34f ;

Best wishes,

More info from Ansley and Russ:

It turns out that the NetCDF library decides to read the attribute
missing_value as a double-precision attribute, but it reads the
_FillValue as a single-precision attribute.  So, the internal bad-value
that is stored by Ferret matches what is in the single-precision
variable when using ex_float_fill.nc but does not match it in

The correct way to specify the value for an attribute is
with*-1.E+34f*;   to explicitly give the value a data type.  It seems
that because _FillValue has a standardized meaning, the library matches
the attribute's type with the variable it is associated with.  It is not
only the CF standard, but also the NetCDF library conventions which
gives the _FillValue a special meaning. Here,


the description says that _Fillvalue "should be scalar and of the same
type as the variable."  In fact, it seems it is automatically
interpreted as having the same type. A missing_value attribute is
described as a useful attribute whose use may be determined by applications.

Adding the missing value attribute with nco may or may not be a
quick task. If there isn't sufficient space in the definition
part of the file nco may have to make a temporary copy of each
file. Still, in the long term it will probably be worth it to
have CF compliant files.

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