[CF-metadata] COARDS name for a time offset

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 15 Jan 2007 08:29:17 +0000

Dear Ed and John

Ed, the procedure for requesting a new standard_name is simply to post to this
this with a definition.

----- Forwarded message from John Caron <caron at unidata.ucar.edu> -----
> since the swath data occurs at the same lat/lon pixels, it is useful to
> make these into 3D variables:
>
> variable[t, y, x]
>
> which have coordinates:
> lat[y,x]
> lon[y,x]
> time_for_variable[t,y,x]
>
> the times for different variables may be different.
>
> to save space, we want to calculate the time variables, instead of
> storing them, as:
>
> time_for_variable[t, y, x] = reference_time[t] + sst_dtime[t,y,x] +
> time_offset_for_variable
>
> or is it
>
> time_for_variable[t, y, x] = reference_time[t] + sst_dtime[t,y,x] +
> time_offset_for_variable[y,x]

Good question - I may have misunderstood. However, both are plausible, and
other things might be proposed. My question is therefore how much of this we
want to standardise in CF. Would it be sufficient, for instance, to provide
the standard_name of time_offset and for GHRSST to have its own additional
convention on how to combine the 1D time coordinate with the auxiliary time
coordinates? A generic application would then not be able to work it out; it
would use the 1D reference_time as its time coordinate, which is a sensible
thing to do. A GHRSST-aware application would behave differently.

> if so, and we want to proceed, perhaps a "formula" attribute (a la
> grid_mapping) would be a good generalized way to do these calculations?

We could certainly devise such a convention, but I question whether it would
be any more useful than recording the formulae in comment attributes. We only
need a new convention if there is a serious possibility that a generic
application might want to decode and use it, or if this is likely to become
a commonly used convention. Otherwise the information needs only to be human-
readable. What do you think?

Best wishes

Jonathan
Received on Mon Jan 15 2007 - 01:29:17 GMT