Dear John and Rich I think platform_altitude would be fine as a standard name. Altitude means above the geoid. CF does not define a default geoid. At present it is not possible to specify what geoid is being used (if you wish to be precise) but we have discussed this a few times before. I append some stuff which I wrote in an email to Rich and others in May. To implement this would require a trac ticket, proposing to define a geoid_name attribute in Appendix F for the grid_mapping variable. I will propose the ticket if you'd support it.
Best wishes Jonathan * Vertical coordinate variables generally have CF standard_name attributes (standard_names are recommended, though not mandatory). The standard_name defines the vertical coordinate relative to a geophysically described surface e.g. geoid, ellipsoid, mean sea level, surface (= bottom of atmosphere). Hence it would be redundant to identify the vertical datum in a geophysical way in any other part of CF metadata. I mean, for instance, CF does *not* have vertical coordinate variables of "height" generically. The "height" is always defined as being wrt a geophysical surface. (The standard_name of height means specifically height above the surface i.e. land or sea surface.) * Some of these special surfaces, especially ellipsoid and geoid, need more precise definitions for some purposes. The existing CF grid_mapping mechanism can define the ellipsoid in terms which translate obviously to WKT terms (see ticket 80 http://kitt.llnl.gov/trac/ticket/80. (This ticket is agreed but not yet implemented in the CF standard document.) It would be easy, and I think logical, to add an attribute of geoid_name to identify the geoid. ----- Forwarded message from John Caron <[email protected]> ----- > Date: Thu, 18 Sep 2014 14:01:15 -0600 > From: John Caron <[email protected]> > To: "Signell, Richard" <[email protected]> > CC: CF Metadata List <[email protected]>, John Graybeal > <[email protected]> > Subject: Re: [CF-metadata] Why "surface_altitude" instead of > "platform_altitude"? > > As I recall, the original proposal was for station_altitude. We decided to > change "station" to "platform". At the same time it was thought that the > existing standard name of "surface altitude" would be synonymous. I at > least was thinking of ground stations. So I think we make a mistake there > and "platform_altitude" would be the right correction. > > An altitude of course needs a datum, and I think we have not been clear > enough on that. I think we should review our use or non-use of vertical > datum. A quick look seems to imply that "WGS 84" is assumed (?) > > On Thu, Sep 18, 2014 at 1:42 PM, Signell, Richard <[email protected]> wrote: > > > John, > > So then the surface needs to be defined relative to some known datum, no? > > > > Maybe we need platform_altitude_above_datum and a specification of > > the vertical datum (EPSG:5701 (MSL), EPSG:5703 (NAVD88), etc) > > > > On Thu, Sep 18, 2014 at 1:47 PM, John Graybeal > > <[email protected]> wrote: > > > I assume surface_altitude is an important variable for providing the > > vertical location of measurements relative to a surface (as opposed to > > relative to a geoid -- notwithstanding the definition issue). > > > > > > John > > > > > > On Sep 18, 2014, at 08:30, Signell, Richard <[email protected]> wrote: > > > > > >> Maybe a simpler approach would be to just adopt "platform_altitude" as > > >> an alias for "surface_altitude" and suggest deprecating the use of > > >> "surface_altitude"? > > >> > > >> On Thu, Sep 18, 2014 at 11:15 AM, John Graybeal > > >> <[email protected]> wrote: > > >>> Interesting that there is so little discussion of this language in the > > mail list, only in John Caron's 2011.09.16 mail on standard names for > > stations (which refers to words already in draft 1.6, I think) -- which > > came at the tail end of a long thread on platform names/IDs. > > >>> > > >>> From those words, I infer that the original drafter thought > > surface_altitude was just as good for describing platform location, as it > > was for describing observation location. I suspect the assumption was that > > any corresponding observations were at the same location as the platform. > > >>> > > >>> Since this is not always true, I'm with you that there should be a > > term platform altitude, and it should be the one used in this sentence. > > >>> > > >>> I hereby propose the standard name platform_surface_altitude (m), > > "Standard names for platform describe the motion and orientation of the > > vehicle from which observations are made e.g. aeroplane, ship or satellite. > > >>> The surface called "surface" means the lower boundary of the > > atmosphere. Altitude is the (geometric) height above the horizontal > > reference surface." > > >>> > > >>> Note I've changed the standard wording of the _altitude definition, > > which generally says ".. above the geoid, which is the reference > > geopotential surface. The geoid is similar to mean sea level." This seems > > clearly in conflict with the definition of surface_altitude and this new > > term, and I think it should be changed in surface_altitude's definition too. > > >>> > > >>> I suppose if people agree with you and me, we need to do a Trac ticket > > for the corresponding change to the standard. > > >>> > > >>> John > > >>> > > >>> > > >>> > > >>> On Sep 18, 2014, at 06:40, Signell, Richard <[email protected]> wrote: > > >>> > > >>>> In the CF-1.6 and CF-1.7 draft doc, in section H.2, we have: > > >>>> > > >>>> "It is recommended that there should be station variables with > > >>>> standard_name attributes " platform_name ", " surface_altitude " and > > >>>> ??? > > >>>> platform_id ??? when applicable." > > >>>> > > >>>> Why is this "surface_altitude" instead of "platform_altitude"? > > >>>> > > >>>> In the ocean, we have lots of upward-looking Acoustic Doppler Current > > >>>> Profilers (ADCP), where the instrument with transducer and other > > >>>> sensors is located some distance below the ocean surface. While > > >>>> velocity and other properties are measured in vertical bins above the > > >>>> instrument (timeSeriesProfile), other properties like pressure and > > >>>> temperature are measured at the instrument. > > >>>> > > >>>> Since the instrument is not at the surface, it seems misleading to use > > >>>> the standard_name "surface_altitude" instead of "platform_altitude", > > >>>> particularly when we already have "platform_name" and "platform_id". > > >>>> > > >>>> In this example CF_1.6 ADCP dataset: > > >>>> > > >>>> > > http://geoport.whoi.edu/thredds/dodsC/usgs/data2/rsignell/data/adcp/7201adc-a_cf16.nc.html > > >>>> > > >>>> the variable "platform_altitude" has a value of -10.4522 m: > > >>>> > > http://geoport.whoi.edu/thredds/dodsC/usgs/data2/rsignell/data/adcp/7201adc-a_cf16.nc.ascii?platform_altitude > > >>>> > > >>>> but we are forced to use a standard_name of "surface_altitude". > > >>>> > > >>>> Why not "platform_altitude"? > > >>>> > > >>>> Thanks, > > >>>> Rich > > >>>> > > >>>> -- > > >>>> Dr. Richard P. Signell (508) 457-2229 > > >>>> USGS, 384 Woods Hole Rd. > > >>>> Woods Hole, MA 02543-1598 > > >>>> _______________________________________________ > > >>>> CF-metadata mailing list > > >>>> [email protected] > > >>>> 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 > > > > > > > > > > > -- > > Dr. Richard P. Signell (508) 457-2229 > > USGS, 384 Woods Hole Rd. > > Woods Hole, MA 02543-1598 > > _______________________________________________ > > 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 ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
