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20031202: IDV handling of _FillValue in short integer fields

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  • Subject: 20031202: IDV handling of _FillValue in short integer fields
  • Date: Tue, 02 Dec 2003 16:42:56 -0700
>From: Richard Signell <address@hidden>
>Organization: SACLANTCEN
>Keywords: 200312021045.hB2Aj7p2025342

Hi Rich-

>I've got a CF-1.0 compliant file (attached) where I'm packing the data
>as
>short integers to save space.  

Good idea.

>As I understand it, the CF Convention should read the _FillValue 
>(written with the units of the "packed" variable, in this case
>short integer) and use it to indicate missing values, which should
>not be used in the "display" chosen.
>
>In the attached file, there is a "float" field called "depth" 
>and a packed "short integer" field called "hsig" (significant
>wave height).   
>
>IDV correctly eliminates the _FillValue elements
>from "depth" in the display, but does not eliminate the _FillValue
>elements in the "hsig" field.   (I was doing color shaded plan view
>displays).  
>
>Happily, the good data values are displayed as correctly
>scaled, so IDV is using the "scale_factor" and "add_offset" values
>correctly.  

The difference is that hsig has both _FillValue and scale_factor
and add_offset, whereas depth only has the _FillValue attribute.
This is causing some irregularity as you noticed where the
_FillValue points are not getting set to missing values.  Your
file is okay, it's our code that's broken. ;-)

>This improper handing of _FillValue isn't a major bug, I guess, 
>but I thought I should report it.

Thanks for pointing this out.  We've determined the problem
and are working on the best way to solve it.  I'll send more
when I have the answer, but just wanted to let you know
we're working on it.

>BTW, "ncview" displays both fields correctly
>(without displaying the _FillValues).

Good info to have.  

Don Murray
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