Hi all -
> From a CDM developer perspective, an auxiliary coordinate is "just as
> good" as a regular coordinate variable. The extra requirements on
> coordinate variables are helpful in knowing when to optimize, eg
> monotonicity allows one to efficiently find the index given the
> coordinate value.
Are we reaching agreement that fill values are allowed in auxiliary
coordinate variables, aside from
> 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).
Should we be discussing this on the trac ticket? That would provide a
better trail - Mailman isn't very good if you're searching for a thread
like this - the subject doesn't mention coordinates.
Thanks -
Nan
>
> On 3/26/2012 10:05 AM, Jim Biard wrote:
>> 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
>> <mailto: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 <tel:%28508%29%20289-2444> *
>> *******************************************************
>>
>>
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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 <mailto:jim.biard at noaa.gov>
>> 828-271-4900
>>
>>
>>
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>
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* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
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Received on Mon Mar 26 2012 - 11:41:46 BST
