Karl, My point is that putting the reference surface in the standard name (potentially) proliferates standard names for things that (like temperatures in different units) are not different except for their reference frame. I agree that we don’t want to put the datum information in the units, but the place for this sort of information already exists - it’s the grid_mapping variable. We don’t have the appropriate attributes defined yet, but that is where the information should go. The definition of the standard name can state a requirement for the information to present in a grid_mapping variable. I thought that the point of standard names was to assist in identifying variables that are of the same kind or class, and that the goal was to avoid putting implementation details into standard_names. By tacking on CRS details, it seems to me that we are moving away from that goal.
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: [email protected] o: +1 828 271 4900 On Feb 11, 2014, at 1:44 PM, Karl Taylor <[email protected]> wrote: > Hi all, > > for temperature, the units imply a zero point, but for height they don't. > For time, we specifically include the zero point in the units (e.g., "days > since 20010101") and we also distinguish among various calendars with the > "calendar" attribute. For height I wouldn't advocate that approach, but > rather the already proposed hybrid approach: the standard name (roughly) > specifies the reference surface, which can then be more precisely defined in > another place (e.g., within the grid_mapping). > > best regards, > Karl > > > > > On 2/11/14, 10:05 AM, Jim Biard wrote: >> 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: [email protected] >> o: +1 828 271 4900 >> >> >> >> >> On Feb 11, 2014, at 11:43 AM, Jonathan Gregory <[email protected]> >> 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 >>> [email protected] >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >> >> >> >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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