[Ken, Craig -- a quick look please?]
As a general rule level 3 satellite fields are "representative" -- or
perhaps "accumulated" would be an alternative term. They are the
accumulated result of a number of satellite passes that take place at
distinct times, and then perhaps some interpolations, smoothing or
other post-processing applied. Arguably more detailed CF conventions
are appropriate -- beyond simply "bounds" and cell_methods on the time
axis -- in order to capture the timing of the swaths that go into the
final field.
The GHRSST project comes to mind as a group that would have dealt with
this question. I see that the OSTIA home page says the GHRSST data
standards are used. So I've cc'ed a couple of people from the GHRSST
project to see if they'd care to comment on treatments of this question.
- Steve
=================
Jonathan Gregory wrote:
> Dear Jon
>
> CF doesn't provide a way to do this except by giving bounds. I think that's
> the right thing to do, because the length of the interval alone doesn't say
> when it starts and stops, which applications may need to know.
>
> The cell_methods indicates how the value represents the variation within the
> interval. For an intensive quantity, "point" is the default i.e. instantaneous
> in time. To indicate a mean, cell_methods of "mean" should be specified. You
> are saying it is "representative" in some vaguer way than a mean, and it is not
> instantaneous. That sounds like a different cell_methods. Perhaps it would be
> a good idea to allow "cell" to be specified in cell_methods for intensive
> quantities, to indicate a "representative" value in this vague sense. ("cell"
> is the default cell_methods for an extensive quantity, which relates to the
> entire cell and depends on its size.) I think this vagueness should in general
> be discouraged; it would be better to be more precise and specify "mean",
> "median" etc., but if you can't be precise it'd be nice to be able to say so.
>
> What do you think? That would require a small change to the convention.
>
> Cheers
>
> Jonathan
>
>
>> We have many datasets for which we need to express the precision of the
>> time axis. For example, the OSTIA sea surface temperature dataset
>> contains daily fields. The data are considered "representative" of a
>> particular day, without necessarily being a simple average over the day.
>> At the moment the data are registered to 12:00Z on each day, but this is
>> indistinguishable from an instantaneous snapshot at this time.
>>
>> I guess it would be possible to express the temporal precision using the
>> "bounds" attribute for the variable in question
>> (http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.ht
>> ml#cell-boundaries), by specifying the start and end of each day as the
>> bounds. Is there a less verbose way of providing this information,
>> perhaps by stating the precision as "1 day/24 hours/whatever" as a
>> single attribute?
>>
>> Jon
>>
>> --
>> Dr Jon Blower
>>
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