Hi. It seems to me that tacking on a description of the datum in the standard name isn’t a good plan. It creates a linkage between standard names and grid mappings / WKT blocks. The nature of the height of the sea surface is not altered by the choice of datum. The values will be different, depending on what sort of height, but you can (most of the time!) translate heights from one CRS to another. It is definitely more complicated, but tacking on a datum description appears to me to be akin to having different standard names for “temperature_in_C” and “temperature_in_K”. If you have properly specified your CRS, the question of where the zero in your height scale is located is completely unambiguous.
Grace and peace, Jim Visit us on Facebook Jim Biard Research Scholar Cooperative Institute for Climate and Satellites NC North Carolina State University NOAA's National Climatic Data Center 151 Patton Ave, Asheville, NC 28801 e: jbi...@cicsnc.org o: +1 828 271 4900 On Feb 11, 2014, at 11:43 AM, Jonathan Gregory <j.m.greg...@reading.ac.uk> wrote: > Dear Rich > >> 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? > > Yes, that is the way we've been proceeding up to now. In fact we do not have > any stdnames yet referring to tidal datum, but you're welcome to propose them > if they're needed now, of course. > >> geoid: NAVD88, GEOID93, GEOID96, USGG2009, etc >> ellipsoid: WGS84, Airy 1830, Airy Modified 1849, etc >> tidal_datum: MLLW, MLW, MTL, MHW, MHHW, etc > > Thanks for these useful lists! I would tend to think that we should > give different standard names for the various different tidal datums, since > I would regard those as different geophysical quantities - would you agree? > If there was data which referred to a tidal datum but didn't actually know > which one it was, I suppose it might still be useful (if imprecise) and it > could have a standard name that referred to "tidal datum" generically. But > if you know it's mean_high_water (for instance), I would spell that out in > the standard name. > > However I think the various geoids are all different estimates of the same > geophysical quantity, so they should *not* have different standard names. > Likewise the ref ellipsoid is the "best" ellipsoid approximating the geoid - > again, that is a single geophysical concept, with many alternative versions. > > So we need a place to name the geoid, if that is the vertical datum. It would > be good to have a similar treatment to CRS WKT for this, but I don't see a > place in WKT where the geoid can be identified. Can anyone help? Is the geoid > implied by, or identical to, the vertical datum name, perhaps? How does one > know, in WKT, whether the vertical datum is a geoid or a ref ellipsoid? > > Best wishes > > Jonathan > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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