[CF-metadata] [Fwd: Re: Problems reading NetCDF data - get NaNs]

[Steve Hankin]

Jon,

It certainly does look like the wording as-is (as you have quoted) is 
self-contradictory.  But the phrase "units of the packed values", while 
intuitively clear, is actually a mis-statement, since "units" do not 
really apply to the packed data.  The right wording is tough to find ....

How about

    "When a variable uses a packed encoding (i.e. the variable has
    scale_factor and/or add_offset attributes -- see section 8.1)
    missing values are interpreted in the packed encoding.  In other
    words, grid cells with missing values are detected before the data
    are unpacked."

Not entirely happy with this, either ... 

    - Steve

=========================================

John Caron wrote:
> A user has misread section 2.5.1 on how missing values interact with 
> scale/offset, resulting in incorrectly written files. We might want to 
> consider some rewording on this in the future.
>
> Here is the relevent conversation:
>
>>> "The missing values of a variable with scale_factor and/or 
>>> add_offset attributes (see section 8.1) are interpreted relative to 
>>> the variable's external values, i.e., the values stored in the 
>>> netCDF file. "
>>>
>>> If I change this in my head to
>>>
>>> "The missing values of a packed variable are in the units of the 
>>> packed values, i.e., the values stored in the netCDF file."
>
>> I was confused by the "relative to the variable's external values".
>> In my mind the "external values" are the ones that an external app
>> sees, i.e. the unpacked values.  Your interpretation makes perfect
>> sense though.
>
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-- 
--

Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
7600 Sand Point Way NE, Seattle, WA 98115-0070
ph. (206) 526-6080, FAX (206) 526-6744

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