[CF-metadata] fixed sensors, depth, datum

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 16 Sep 2008 12:32:55 +0100

Dear Jon

> > - height relative to the ellipsoid
> > - height relative to the geoid
>
> From a coordinate referencing point of view they both
> give height (in units of length) above a reference surface, but the
> surfaces happen to be different. Do you consider them geophysically
> distinct because "height relative to geoid" is a close approximation
> to potential energy, whereas "height relative to ellipsoid" is not?
> Similarly, the geoid approximates sea level whereas the ellipsoid does not?

Yes, I would say that distinctions like that make them different quantities.
Plain "height" (above the surface) is likewise a different geophysical
quantity. However, I think that specification of geoid or ellipsoid is not part
of the identity of the geophysical quantity. It is more in the nature of an
extra parameter for working out the numbers, like the reference pressure used
for potential temperature. Of course, I realise this is somewhat arbitrary.

Best wishes

Jonathan
Received on Tue Sep 16 2008 - 05:32:55 BST