Dear Jonathan et al,
> - height relative to the ellipsoid
> - height relative to the geoid
> In my opinion these are distinct geophysical quantities.
I don't quite understand why these quantities are distinct
geophysically. 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?
Regards,
Jon
On Mon, Sep 15, 2008 at 6:32 PM, Jonathan Gregory
<j.m.gregory at reading.ac.uk> wrote:
> Dear all
>
> Referring to Dale's list of requirements
>
> - height relative to the ellipsoid
> - height relative to the geoid
> In my opinion these are distinct geophysical quantities. As Dale pointed out,
> we already have the standard name of "altitude" for the second of them, and
> the first should probably have the name height_above_reference_ellipsoid for
> consistency with other standard names. That would need to be proposed to this
> email list.
>
> - name of the ellipsoid
> - name of the geoid
> These could be added as possible attributes to the grid_mapping variable.
> That is a conventions change that would need a trac ticket to propose and
> discuss. In fact Phil Bentley did propose to have such attributes in his
> CRS ticket. We had discussions in which I expressed my usual disquiet about
> redundancy. What if the name of the ellipsoid is not consistent with the
> parameters of the ellipsoid that have been supplied, if they have been? I
> am not sure how to resolve that. This concern does not apply to the geoid,
> because it is not practical to specify it in detail as part of CF metadata;
> we can only identify it by name. If the ellipsoid_name is a grid_mapping
> attribute too, it simplifies Dale's example, as it doesn't have to appear
> in its own dummy variable.
>
> - altitude of (offset to) other common datums (e.g. MLLW)
> Dale's offsets are single numbers, but they could in some circumstances be
> (lon,lat) fields, as Ethan says. In either case, they can be data variables,
> so I see no problem that that. The other common datums are tidal datums, for
> instance. Again, I think the various tidal levels are distinct geophysical
> variables, and need different standard names to be proposed
> e.g. altitude_of_mean_lower_low_water and altitude_of_mean_low_water.
>
> I think I don't properly understand how these concepts relate to concepts
> like "Australian height datum", which Ethan mentions. What does that mean
> exactly, can someone explain? I suspect it must mean the designation of a
> particular geoid, and saying that a particular location has a certain altitude
> wrt that geoid. By specifying a geoid, a location and an altitude you would
> thereby define altitude for everywhere else in the world. Is that right?
>
> Best wishes
>
> Jonathan
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--
Dr Jon Blower
Technical Director, Reading e-Science Centre
Environmental Systems Science Centre
University of Reading
Harry Pitt Building, 3 Earley Gate
Reading RG6 6AL. UK
Tel: +44 (0)118 378 5213
Fax: +44 (0)118 378 6413
j.d.blower at reading.ac.uk
http://www.nerc-essc.ac.uk/People/Staff/Blower_J.htm
Received on Tue Sep 16 2008 - 02:40:31 BST
