missing_value attribute

From: Dave Allured <dave.allured_at_nyahnyahspammersnyahnyah>
Date: Tue Oct 11 2011 - 12:13:26 MDT


As of CF conventions 1.5, October 2010, the missing_value attribute
is recognized as a valid alternate for _FillValue. _FillValue is
preferred, but missing_value is no longer deprecated.


So it is important for CF as well as COARDS, that NCL continues to
handle the missing_value attribute when reading Netcdf files, as you
described in [5] below. Converting to _FillValue for NCL internal
use is a reasonable way to do this.


On 10/11/2011 6:23 AM, Dennis Shea wrote:
> [1] _FillValue is what the CF convention states should be used
> to indicate missing values. Further, Unidata says the same thing.
> NCL recognizes _FillValue only.
> [2] missing_value is a COARDS convention attribute. It has no
> special status under CF or within NCL.
> [3] _MissingValue, etc....., mean nothing to any known software
> [4] By rule, the type of the _FillValue should be the same as
> the variable with which it is associated. However, there is
> nothing in the netCDF software to prevent this from happening.
> [5] NCL does try to be 'helpful' in the following way. When a variable
> is read and NCL sees it has a missing_value attribute but
> no _FillValue, NCL will assign an virtual _FillValue' attribute.
> NCL checks the type of the missing_value attribute and the type
> of the associated variable. If they are the same, NCL creates
> the _FillValue attribute; if the are different and NCL can
> coerce the missing_value to the type of the variable, NCL will
> do it. If not, you get the warning message indicated earlier.
> This is *not* an NCL problem. The file creators 'screwed up'.
> NCL can be used to address the issue but it must be done manually.
> [6] NCL can check for NaNs and replace the values.
> http://www.ncl.ucar.edu/Document/Functions/Built-in/isnan_ieee.shtml
> http://www.ncl.ucar.edu/Document/Functions/Built-in/replace_ieeenan.shtml
ncl-talk mailing list
List instructions, subscriber options, unsubscribe:
Received on Tue Oct 11 12:13:38 2011

This archive was generated by hypermail 2.1.8 : Wed Oct 12 2011 - 15:59:45 MDT