[CF-metadata] Vertical datums (again)
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-- John Helly, University of California, San Diego / San Diego Supercomputer Center / Scripps Institution of Oceanography / 760 840 8660 mobile / stonesteps (Skype) / stonesteps7 (iChat) / http://www.sdsc.edu/~hellyj On 2/14/14 5:13 PM, Eizi TOYODA wrote: Hi Jim and all, Basically I support your findings on the word usage. If asked, the geometric height in meteorology should be the orthometric height in geodesy, but I'm also aware that most meteorologists won't mind to mix it up the concept with the geodetic height. The difference of geopotential and geometric height is the scale which is proportional to gravity acceleration, so the third digit may be different. The difference between geodetic and orthometric height is origin. Best, Eizi -- TOYODA Eizi, Japan Meteorological Agency 2014/02/15 5:39 "Jim Biard" <jbiard at cicsnc.org>: Hi. So, I went through and looked more closely at the standard name table, and I have a few comments. But first, the results of some googling that I did. When you look at the geodesy literature, there are multiple kinds of heights. Geoidal height is the height of a point on a geoid relative to a reference ellipsoid. Geodetic height is the height of a point on the surface relative to a reference ellipsoid. Orthometric height is the height of a point on the surface relative to a reference geoid. Geodesists also use the word elevation when referring to geodetic or orthometric heights. They don't usually talk about heights above the surface, but that's not their area of interest. I found that in meteorology, geometric height (as opposed to geopotential height) appears to be what geodesists call orthometric height, or height relative to a geoid. I also discovered that in aviation, they often use height to mean "height above the surface", and altitude to mean "height above the vertical reference". In addition to all of the above, I found that there appears to be no consensus on specific meanings for height, elevation, and altitude. Height appears to be generally accepted (outside of aviation) as any measure of vertical distance. Elevation and altitude both tend to be heights measured relative to a reference surface. Geodesists use the three terms interchangeably. Because the word height does not usually invoke any particular reference frame, it is mostly used with qualifiers (orthometric, above sea level, etc). In CF, the standard name table has defined altitude and height. Height is defined to be "height above the surface". Not my preference, but it's a done deal. Altitude is defined as orthometric height. It uses the word "geometric", which seems to be the way meteorologists tend to refer to it. We then have a lot of qualified height names, some of which are themselves definitions of measures from geodesy, some of which use height in the sense of "height above the surface", and some of which use height in the sense of "height above a vertical reference". Interestingly, we don't have standard names "elevation" or "height_above_geoid". This all makes sense for a system that appears to have grown from one where vertical datums weren't really considered (horizontal ones, either!), into one where questions of reference frames have become more important. Given all the confusion of usage both within CF and in the community at large, I guess there's no reason not to continue to proliferate height qualifiers in standard names. We already have several. I'd be interested to know how often people have used the standard name "height" to mark vertical coordinates, when the values they have almost certainly are not "height above the surface". Grace and peace, Jim <http://www.cicsnc.org/> Visit us on Facebook <http://www.cicsnc.org/> 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: jbiard at cicsnc.org o: +1 828 271 4900 <http://www.cicsnc.org/> <http://www.cicsnc.org/> <http://www.cicsnc.org/> <http://www.cicsnc.org/> On Feb 12, 2014, at 11:42 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote: <http://www.cicsnc.org/> <http://www.cicsnc.org/> Dear Jim I think the same as Karl. The reason is that I regard height above geoid and height above reference ellipsoid as different geophysical quantities, as are height above bedrock (the example I gave in an earlier email, to Rich) and height (in the sense of the CF standard_name table i.e. above the ground). If I am standing on the summit of Mount Everest, the height above ground of my crampons is about 0 m, but their altitude is about 8848 m. In fact in your email you referred to them as different "sorts" of height. That sounds to me like different geophysical quantities. Conversion between them is possible but it is a non-trivial operation involving geophysical data. Similarly conversion is possible between altitude and geopotential height but they are different geophysical quantities with different standard names. The geoid and reference ellipsoid definition, however, are fine in the grid mapping, because they do not define the geophysical quantity. They just make it numerically precise. Best wishes Jonathan ----- Forwarded message from Jim Biard <jbiard at cicsnc.org> ----- <http://www.cicsnc.org/> From: Jim Biard <jbiard at cicsnc.org> Date: Tue, 11 Feb 2014 13:51:56 -0500 To: CF metadata <cf-metadata at cgd.ucar.edu> X-Mailer: Apple Mail (2.1827) CC: Karl Taylor <taylor13 at llnl.gov> Subject: Re: [CF-metadata] Vertical datums (again) 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: jbiard at cicsnc.org o: +1 828 271 4900 On Feb 11, 2014, at 1:44 PM, Karl Taylor <taylor13 at llnl.gov> wrote: <http://www.cicsnc.org/> 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: <http://www.cicsnc.org/> 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: jbiard at cicsnc.org o: +1 828 271 4900 On Feb 11, 2014, at 11:43 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote: <http://www.cicsnc.org/> Dear Rich <http://www.cicsnc.org/> 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? <http://www.cicsnc.org/> 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. <http://www.cicsnc.org/> geoid: NAVD88, GEOID93, GEOID96, USGG2009, etc ellipsoid: WGS84, Airy 1830, Airy Modified 1849, etc tidal_datum: MLLW, MLW, MTL, MHW, MHHW, etc <http://www.cicsnc.org/> 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 at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> _______________________________________________ CF-metadata mailing list CF-metadata at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> _______________________________________________ CF-metadata mailing list CF-metadata at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> <http://www.cicsnc.org/> <http://www.cicsnc.org/> _______________________________________________ CF-metadata mailing list CF-metadata at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list CF-metadata at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> <http://www.cicsnc.org/> _______________________________________________ CF-metadata mailing list CF-metadata at cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> <http://www.cicsnc.org/> _______________________________________________ <http://www.cicsnc.org/> CF-metadata mailing list <http://www.cicsnc.org/> CF-metadata at cgd.ucar.edu <http://www.cicsnc.org/> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata <http://www.cicsnc.org/> <http://www.cicsnc.org/> -------------- next part -------------- An HTML attachment was scrubbed... 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