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
Received on Wed Jun 02 2010 - 09:10:07 BST
