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Re: GDS - LAS interaction
Hi folks,
A little metadata question for you: based on the information here:
http://ferret.wrc.noaa.gov/noaa_coop/coop_cdf_profile.html
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:
http://lis1.sci.gsfc.nasa.gov:8080/las/servlets/dataset
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
page:
http://ferret.pmel.noaa.gov/Ferret/LAS/FAQ/ingesting_data.htm
where they used the "missing_value" tag. I do not know if it
is relevant or not.
Could you please take a look?
Thanks.
Yudong
Joe Wielgosz / joew@cola.iges.org
---------------------------------------------------
Center for Ocean-Land-Atmosphere Studies (COLA)
Institute for Global Environment and Society (IGES)
http://www.iges.org
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