Jonathan, Thanks for the detailed explanation (and analogy) of why it's useful to tack on the "_above_geoid" or "_above_ellipsoid" or "_above_tidal_datum" to the standard-name.
So we do that and then specify the geoid, ellipsoid or tidal datum elsewhere in the grid_mapping variable, right? geoid: NAVD88, GEOID93, GEOID96, USGG2009, etc ellipsoid: WGS84, Airy 1830, Airy Modified 1849, etc tidal_datum: MLLW, MLW, MTL, MHW, MHHW, etc Older bathymetry data are almost always reported relative to tidal_datums. Yes, this is a huge can of worms. It's why vertical datum software such as http://vdatum.noaa.gov/ are popular, so that folks can convert from something like MLLW to NAVD88. -Rich On Mon, Feb 10, 2014 at 1:30 PM, Jonathan Gregory <j.m.greg...@reading.ac.uk> wrote: > Dear Jim and Rich > > Many thanks for your helpful comments. I see a prospect of my understanding > things a bit better than before! > > Jim says that a vertical datum always has a reference ellipsoid. Sometimes a > vertical datum might *be* a reference ellipsoid. Sometimes it is a geoid, and > in that case, is it accompanied by a reference ellipsoid as part of the > definition of the vertical datum? > > Rich comments that a vertical datum could be orthometric. If I've understood > Jim correctly, orthometric describes how you measure the height wrt the > reference surface. It is not a third type of surface, in addition to geoid > and reference ellipsoid. Is that right? > > Tides define a different sort of reference surface from geoid and ellipsoid. > Are there also vertical datums which involve tidal levels in their definition? > >> why can't we just say >> "sea_surface_height_above_datum" or just "sea_surface_height" and then >> specify the vertical datum, no matter what it is? > > I don't think we should do so because height wrt geoid and height wrt > ellipsoid > are rather different quantities. For that reason they have different standard > names (altitude and height_above_reference_ellipsoid, and there is also a > standard name of geoid_height_above_reference_ellipsoid). They are seriously > different in value, aren't they? - by 100s of metres, so you have to know > which > one you are dealing with. If they had the same standard name, a height wrt > geoid from one data source and a height wrt ref ellipsoid from another might > be regarded as comparable quantities, which could be a serious error. Of > course > I recognise that the stdname is not the only metadata one should consult, but > it is the first point of call. > > To make an analogy, suppose we just defined height as "vertical distance above > something", with something defined elsewhere. Then altitude and height above > sea floor would be synomymous standard names. I don't think that would be as > helpful to the data-analyst. > > I do think, however, that it's acceptable to define the geoid or reference > ellipsoid in another place (the grid mapping) from the standard name. This is > still a risk, because heights on different vertical datums might be treated as > comparable they aren't, but on the other hand there are cases where heights on > different vertical datums could be compared e.g. if they come from models with > a different shape for the Earth. > > We can meet Rich's need, I think, if we provide a way for the grid_mapping to > specify vertical datums which involve the geoid being implied. > > Best wishes > > Jonathan > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata -- Dr. Richard P. Signell (508) 457-2229 USGS, 384 Woods Hole Rd. Woods Hole, MA 02543-1598 _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata