Dear Jonathan and Ethan (et al),
Definitely a head-scratcher, this one. My own view (which may well
change!) is as follows:
- If we were concerned only with data expressed as discrete vertical
profiles then I would tend to agree with Ethan (the distinction is
just a vertical coordinate transformation).
- However, given that we are primarily concerned with 3D fields I
think that the quantities are distinct (agreeing with Jonathan). A
x-y slice through the field has a specific geophysical meaning if the
vertical coordinate is height_above_geoid, but it has no particular
geophysical meaning if the vertical coordinate is
height_above_ellipsoid.
Two points to muddy the waters further:
- Being pedantic, two points at the same height above the geoid might
not have quite the same potential energy. The 3D geopotential
contours are not everywhere equally-spaced. However, I imagine this
is not usually a large effect, unless the data in question are close
to a large gravity anomaly.
- Vertical coordinate values (heights, altitudes etc) are often
inferred from other quantities (esp. pressure in both air and water).
Since CF is expanding to include in situ data, can we express this
somehow, so that users know that the coordinate value depends on
certain assumptions?
Cheers, Jon
On Mon, Sep 29, 2008 at 9:10 AM, Jonathan Gregory
<j.m.gregory at reading.ac.uk> wrote:
> 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
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
--
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 Mon Sep 29 2008 - 04:18:41 BST
