[CF-metadata] ungridded data

From: Jonathan Gregory <jonathan.gregory>
Date: Wed, 19 Jun 2002 09:16:35 +0100

Dear Bob

> I don't think of station data and trajectories as having boundary
> information, instead being defined at specific lat/lon/time points with no
> associated cells.

Observational station and trajectory data would probably be at points and have
no boundaries, but corresponding model-generated data would be from particular
3D gridboxes, which do have boundaries. In fact when our model (i.e. the UM)
generates timeseries corresponding to stations, it does attach boundary
information to it. Hence I think these kinds of data can have boundary info.

As you say, data on a regular grid which could have boundary info does not
necessarily have it (although it may be recommended) - it depends on whether
the data writer has stored it. Using the presence of boundary info therefore
seems to be an unsatisfactory criterion on both grounds for distinguishing
between what you called "gridded" and what you consider ungridded.

> I guess part of the exercise is to reach an agreed upon terminology!
Indeed. As Brian's reply shows, an alternative definition of "gridded" includes
anything which is discretised. In that case, gridded doesn't imply independent
sets of x and y coordinates, with the valid points being at any choice of x
and y from the sets.

Rather than introducing more metadata at a "higher level", which would be
redundant in principle, I think it would be better to deduce what you need
from the metadata we already have. It's not quite clear to me what the
distinction is between a GriddedVariable and other variables, but if we're
talking about lon-lat-vertical, it sounds as though a GriddedVariable is
one which has dimensions of lat, lon and vertical. Station data will not have
lat and lon dimensions; it will have some "site" dimension instead, with
lat and lon as auxiliary coordinate variables. A trajectory in 3D will not
have lat lon or vertical dimensions, but probably all three will be
auxiliary coordinates with a time dimension. It would be quite easy to make
tests like this. Is that the right distinction?

Cheers

Jonathan
Received on Wed Jun 19 2002 - 02:16:35 BST