[CF-metadata] fixed sensors, depth, datum

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 29 Sep 2008 09:10:36 +0100

Dear Ethan

I agree that different definitions of the reference ellipsoid do not constitute
different geophysical quantities. Likewise different definitions of the geoid
all give the same geophysical quantity. Therefore I agree that the geoid should
be identified as part of the CRS (naming it in the grid_mapping would be
convenient), just as the ellipsoid is identified as part of the CRS (we added
the parameters specifying the ellipsoid to the grid_mapping as part of Phil
Bentley's change to the conventions). I agree too that the definition of the
vertical CRS is relevant both to coordinate variables and data variables. That
is another reason why it would make sense to put it in the grid_mapping.

I do not agree that the geoid and the ellipsoid are geophysically equivalent.
It is quite likely that you might want to have data variables in the same file
for both height above geoid and height above ellipsoid, just as you might also
want to have height above the surface and height above mean sea level. These
are all heights wrt to surfaces which are defined as a function of lat and lon.
All of these surfaces therefore depend on the horizontal CRS, as you say. But
these surfaces are all geophysically distinct. The reference ellipsoid is
"just" a matter of definition, but the others (geoid, surface = bottom of atmos
and mean sea level) are not matters of definition: they are complicated facts
about the real world that have to be measured. Wikipedia says
"The geoid surface is irregular, unlike the reference ellipsoids often used to
approximate the shape of the physical Earth, but considerably smoother than
Earth's physical surface."

These surfaces have different physical meanings. For instance, surfaces with
constant height above the geoid (geopotential surfaces) are those on which
there is zero gravitational/centrifugal force; this not true of other surfaces.
Height above these various surfaces has different geophysical meaning. You
would not want to replace height above geoid with height above ellipsoid by
changing the definition of the CRS. They should remain distinct quantities,
regardless of the definition of geoid and ellipsoid in the CRS. Hence I think
they need different standard names.

Best wishes

Jonathan
Received on Mon Sep 29 2008 - 02:10:36 BST