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 <caron at ucar.edu> -----
> Date: Thu, 18 Sep 2014 14:01:15 -0600
> From: John Caron <caron at ucar.edu>
> To: "Signell, Richard" <rsignell at usgs.gov>
> CC: CF Metadata List <cf-metadata at cgd.ucar.edu>, John Graybeal
> <john.graybeal at marinexplore.com>
> 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 <rsignell at usgs.gov> 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
> > <john.graybeal at marinexplore.com> 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 <rsignell at usgs.gov> 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
> > >> <john.graybeal at marinexplore.com> 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 <rsignell at usgs.gov> 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
> > >>>> CF-metadata at 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
> > >
> >
> >
> >
> > --
> > Dr. Richard P. Signell (508) 457-2229
> > USGS, 384 Woods Hole Rd.
> > Woods Hole, MA 02543-1598
> > _______________________________________________
> > CF-metadata mailing list
> > CF-metadata at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >
> _______________________________________________
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----- End forwarded message -----
Received on Fri Sep 19 2014 - 07:37:01 BST
