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Re: GDS - LAS interaction

Hi folks,

A little metadata question for you: based on the information here:
in particular, the following:

The NOAA cooperative standard does not endorse any particular interpretation of the distinction between missing_value and > _FillValue.
it seems that COARDS-compliant client applications should recognize both the '_FillValue' and 'missing_value' attributes. However, it sounds like from what Luther is saying as if LAS only handles 'missing_value'. Is that correct?

I have a memory of reading at some point that missing_value was being deprecated in favor of _FillValue, which is why I chose to use the latter form in the GDS metadata. However I can't find that page now.

Also, it seemed that _FillValue could be used to do certain optimizations:

_FillValue - If a scalar attribute with this name is defined for a variable and is of the same type as the variable, it will be subsequently used as the fill value for that variable. The purpose of this attribute is to save the applications programmer the work of prefilling the data and also to eliminate the duplicate writes that result from netCDF filling in missing data with its default fill value, only to be immediately overwritten by the programmer's preferred value. This value is considered to be a special value that indicates missing data, and is returned when reading values that were not written. The missing value should be outside the range specified by valid_range (if used) for a variable. It is not necessary to define your own _FillValue attribute for a variable if the default fill value for the type of the variable is adequate.
Now, re-reading this carefully, it seems like the optimizations may only apply when writing to a dataset. If so, then, for read-only OPeNDAP datasets it is pretty much immaterial whether "missing_value" or "_FillValue" is used.

So, can someone who is familiar with COARDS confirm whether GDS should be using missing_value, or LAS should be supporting _FillValue, or both?

- Joe

On Wednesday, September 3, 2003, at 12:05 PM, Yudong Tian wrote:

Hi Jennifer,
   When we added our GDS datasets to the LAS server, we found a
weird thing: the LAS server treats the "UNDEF 9.999E20" as real
data values, so the scale of the plots is screwed up. You can see
this by looking at the scale bar of any generated plots:


  It seems to us that LAS does not recognize the "_FillValue"
tag in the .das files. The LAS people asked us to check out this
where they  used the "missing_value" tag. I do not know if it
is relevant or not.
  Could you please take a look?


Joe Wielgosz / joew@cola.iges.org
Center for Ocean-Land-Atmosphere Studies (COLA)
Institute for Global Environment and Society (IGES)

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