[CF-metadata] Vertical datums (again)

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 17 Mar 2014 16:39:21 +0000

Dear Jim

Yes, I suppose you're right, on reflection. I was thinking of a situation in
which you were interested in the ellipsoid only for the sake of a vertical
datum, rather than as a horizontal datum (which is its usual purpose in
grid_mapping). However, I now understand from the discussion and explanations
from Jon Blower why you cannot actually refer heights to a vertical datum
without implying a particular latitude-longitude coordinate system. So the
existing grid_mapping_name of latitude_longitude will be appropriate if you
want to identify the geoid or ellipsoid as a vertical datum, I agree.

My understanding is now that the height is taken perpendicular to the surface,
and the lat-lon is defined on the surface, so the lat-lon of the point whose
height is being measured depends on the choice of surface. Please correct me
if that's not right.

Cheers

Jonathan

> I thought this was already defined. In the second paragraph of section 5.6, it says that if you aren?t specifying a projected coordinate system (or, I assume, a Cartesian coordinate system such as ECF), then use the name ?latitude_longitude?. I haven?t noticed anything we?ve talked about that would invalidate this usage. We are talking about adding vertical datum specifications and such as further attributes to the variable, but even latitude and longitude values can shift depending on the ellipsoid and/or geoid being used, so these should specified even when there is no projected coordinate system.
Received on Mon Mar 17 2014 - 10:39:21 GMT