I am working with satellite data, and I, for example, have timestamps that
arrive in the data stream along with sensor measurements. I can have
independent missing values in both my time variable and my measurement
variables. I want to preserve all the incoming data, and the restriction
on "true" coordinate variables having no missing values prevents me from
designating my time variable as a coordinate variable. If I want to
represent the relationship between the time variable and the measurement
variables, the only recourse I have is to designate the time variable as an
auxiliary coordinate variable in my measurement variables.
In the observational world, you cannot always be assured of meeting all the
restrictions imposed on "true" coordinate variables. In fact, other
restrictions imposed on coordinate variables might not be met
(monotonicity, for example), even though the contents of the variable do
function properly as coordinate information for another variable. The only
mechanism that I am aware of within CF to document the relationship is the
auxiliary coordinate attribute.
On Mon, Mar 26, 2012 at 11:20 AM, Nan Galbraith <ngalbraith at whoi.edu> wrote:
> Hi Jonathan -
>
> For underway CTD profiles (gliders and floats, too, I'd think) if the
> pressure
> sensor fails intermittently, you can approximate Z by interpolating over
> time, assuming there are good values along the track. In "final" data, we
> might
> do that, but we might like to present raw data in CF files, too. So, yes,
> this data
> would be useful, with fill values here and there in the pressure record.
>
> Are we getting into a grey area that might be outside the scope of the CF
> standard - judgements made about the usefulness of data? Having all your
> coordinates seems like an excellent NetCDF "best practice" and could
> certainly
> be a "rule" for many tools, but it could preempt the use of the CF
> standard for
> some kinds of observational data.
>
> Cheers -
> Nan
>
>
> On 3/26/12 10:48 AM, Jonathan Gregory wrote:
>
>> Dear Nan and John
>>
>> It's a good thing we're having this discussion! In my understanding, we
>> have
>> always prohibiting missing data in aux coord vars, and in section 9 we
>> explicitly allowed for the first time. Evidently we should be clear, one
>> way
>> or the other (which was one of the intentions of the defect ticket I
>> opened).
>>
>> The restriction on coord vars not having missing data is, I think, hard to
>> avoid because they are required to be distinct and monotonic.
>>
>> In Nan's case:
>>
>> For something like towed CTD data, you might have a period of time where
>>> data from the pressure sensor is missing. If neither the coordinate or
>>> aux
>>> coordinate can contain null values, does this mean the only options are
>>> interpolating Z or removing that section of data?
>>>
>> When the sensor is missing, does that mean the data can't be geolocated?
>> As
>> you know, geolocation is one thing CF tries hard to do. Is the data
>> useful if
>> you don't know where it comes from? Perhaps you know in some other way?
>>
>> In John's case
>>
>> Consider a geosynch satellite with lat/lon aux coordinates. the
>>> nature of the image is that there are many points around the outside
>>> of the image that are not located on the earth. i dont see any good
>>> choices other than to put missing values there for lat/lon..
>>>
>> Bert has coincidentally mentioned a similar case (not on the list). I
>> tend to
>> agree that if index space contains locations that simply do not exist, you
>> could put missing data in aux coord vars, but I think this needs a change
>> to
>> the CF convention.
>>
>> To add insult to injury, it seems possible that there are valid data
>>> values at these locations. Not sure about that however. Can anyone
>>> with geosynch data expertise comment?
>>>
>> As in Nan's case, I am curious about how the data can be useful if it
>> doesn't
>> have a location.
>>
>> Best wishes
>>
>> Jonathan
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>>
>>
>
> --
> *********************************************************
> * Nan Galbraith Information Systems Specailist *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 (508) 289-2444 *
> *********************************************************
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>
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--
Jim Biard
Research Scholar
Cooperative Institute for Climate and Satellites
Remote Sensing and Applications Division
National Climatic Data Center
151 Patton Ave, Asheville, NC 28801-5001
jim.biard at noaa.gov
828-271-4900
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