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.
John
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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