[CF-metadata] Vertical datums (again)

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 11 Mar 2014 14:29:25 +0000

Dear David

> I'm a bit confused as to the use of these grid_mapping parameters with
> vertical coordinates - do we need new grid_mapping_names? I'm
> thinking, for example, of linking a geoid specification to a vertical
> "altitude" coordinate.

That's a good point, thanks. I was thinking of the grid_mapping as a place to
record the reference surface in the same way as it does for latitude_longitude,
which was added as a "null" horizontal mapping. No "mapping" is required for
the vertical coordinate; it just needs its reference surface (vertical datum)
to be more precisely specified than its geophysical description of "geoid" or
"reference_ellipsoid" alone, for some applications.

I'd like to suggest we add a new grid_mapping_name of "null" to Appendix F,
thus: "This grid_mapping does not imply any mapping of horizontal or vertical
coordinates. Its purpose is to describe the reference ellipsoid or the geoid."
No map parameters, no map coordinates.

I would further suggest that we make latitude_longitude an alias for null.

It might happen that a different reference ellipsoid is required for vertical
and horizontal coordinates. Thanks to Mark's accepted ticket 70, this will be
no problem; it allows there to be more than one grid_mapping, and the
grid_mapping attribute indicates which coordinate variable each is relevant to.

Best wishes

Jonathan
Received on Tue Mar 11 2014 - 08:29:25 GMT