Re: GDS - LAS interaction

[Joe Wielgosz <joew@xxxxxxxxxxxxx>]

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