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@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata