[CF-metadata] vertical coordinates

From: Jonathan Gregory <j.m.gregory>
Date: Thu, 7 Aug 2008 08:46:34 +0100

Dear Jon et al.

I think there are two separate issues.

(1) I would say that the vertical coordinates which you list

> Depth below the geoid
> Depth below an ellipsoidal approximation of the earth (e.g. WGS84)
> Depth below instantaneous sea surface

are different physical coordinates. They would and should have different
standard names like "depth_below_X". The standard_name of plain "depth"
means "depth below the surface" i.e. the surface which is the bottom of
the atmosphere. That *could* be "instantaneous sea surface" but for
applications where this distinction is important it might be better to use
more precise standard names. Similarly, we already have the following
standard names:

sea_surface_height_above_geoid
sea_surface_height_above_reference_ellipsoid
sea_surface_height_above_sea_level

to make similar distinctions. Introducing more precise standard names for
depth does not require a change to the conventions, just a proposal to be
agreed on the email list, and any quantity can be used as a coord variable.

I tend to think that "reference_ellipsoid" would cover both the real-Earth
ellipsoids and the sphere assumed for models, so they probably do not need
separate standard names. In fact for models it is identical with the geoid
as well.

(2) Precision about *which* real-Earth reference ellipsoid or geoid is meant.
As others have said, this is related to the definition of the horizontal
coordinate reference system. It has been raised several times before, and we
deferred it from the discussion of Phil Bentley's ticket in order to limit the
scope of the discussion and because it appeared no-one had time to formulate
and describe a proposal to deal with the vertical aspects of the datum. As
Seth says, the ellipsoid itself can be described by grid_mapping parameters.
Giving it a name as well might be helpful, but it is redundant and therefore
potentially inconsistent, so there needs to be a clear advantage in doing
this. The geoid model is too complicated to be specified by CF metadata, so I
think it would have to be named in some way.

Cheers

Jonathan
Received on Thu Aug 07 2008 - 01:46:34 BST