[CF-metadata] missing_value vs. _FillValue

From: Karl Taylor <taylor13>
Date: Wed, 05 Nov 2003 08:18:01 -0800

Hi all,

I read in our convention that the missing_value is deprecated and should
no longer be used (use _FillValue instead). What motivated the
deprecation? It seems to me there is a fundamental difference between
the two. My understanding is that the _FillValue is usually used by
netCDF to prefill the disk space, so if the user fails to completely
write all the arrays, but later tries to read in all elements, those
elements unwritten will contain a predictable number (i.e., the
_FillValue). It seems to me that incompletely writing data is in some
sense a mistake, and this mistake can be easily identified by testing
whether the _FillValue is found anywhere in the file. In the common
climate modeling case, on the other hand, some data are "missing" not
because we have made a mistake, but because some quantities are only
calculated in certain regions (e.g., sea surface temperature is
calculated only in ocean regions and is "missing" elsewhere).

I think it is useful to distinguish between the different reasons for
missing data. Because _FillValue is treated specially by netCDF, I
think we should prefer the use of missing_value to identify data that
have been written properly, but are missing by design.

Tell me what danger there is in using a "deprecated" attribute in the
writing of CF-compliant dataset. \

thanks,
Karl

P.S. There is some urgency in addressing this issue since Jean-Yves
Peterschmitt and I are writing a code that makes it easy to write
CF-compliant output contributed to various model intercomparison
projects and for IPCC contributions. Thanks for your thoughts.
Received on Wed Nov 05 2003 - 09:18:01 GMT