Fwd: Re: [CF-metadata] ungridded data

From: Jonathan Gregory <jonathan.gregory>
Date: Mon, 24 Jun 2002 17:55:54 +0100

Dear Bob

> Structurally the two are identical.

As Brian says, they may not be, as the reduced grid may indicate that it is
compressed from a 2D field. However, I suppose that wouldn't necessarily be the
case with all cases where the 2D plane has been covered in a way that has to
represented in 1D because it isn't rectilinear lat-lon.

I think you are setting yourself a hard task to distinguish such a tiling of
the whole surface from an irregular set of scattered points. This is getting
outside the bounds of a purely technical question into the area of scientific
interpretation. Structurally the two are identical because functionally they
are really quite similar. For example, suppose you have a reduced grid covering
the whole Earth i.e. no gaps between its boxes. Then omit a single gridbox. Is
this now a reduced grid with a box missing, or is it a set of scattered points
each with an associated box surrounding it? Or, starting from the other end, it
might be the case that a small set of scattered station data spread over the
world really might be being used to estimate a global average quantity. Sea
level change is estimated from tide gauges in this kind of way. In this
case the user would actually or notionally have associated some kind of
region with each station, so it is not obvious that it would be inappropriate
to guess at the gridbox boundaries.

However, the grid_mapping attribute we have been discussing may help. I suppose
that if there is a grid_mapping, it means that the whole world (or the portion
of it being considered) has been systematically covered in some kind of way.
The application may not understand how the grid_mapping works, but perhaps the
presence of this attribute would be sufficient to decide that the data was a
set of boxes covering the surface, rather than a set of irregularly scattered
points.

Best wishes

Jonathan
Received on Mon Jun 24 2002 - 10:55:54 BST