[CF-metadata] Standard names for CF trac ticket #143

Karl Taylor taylor13 at llnl.gov
Fri May 26 08:55:41 MDT 2017


Thanks Jonathan and Roy for explaining and reassuring me that vagueness 
is o.k. here.  I agree.
best regards,
Karl

On 5/26/17 1:11 AM, Lowry, Roy K. wrote:
>
> Dear Karl,
>
>
> As an oceanographic data centre we come across the term 'mean' sea 
> level quite a lot.  Common understanding of the generic term is that 
> the averaging interval is any period long enough to remove the tidal 
> signal, typically 1-2 years.  This is sufficiently precise for most 
> use cases with the obvious exception of long-term (hundreds of years) 
> studies of sea level variation. For these there are precisely defined 
> sea level averages at fixed points given specific names such as 
> Ordnance Datum Newlyn (averaged from 1915 to 1921) or Malin Ordnance 
> Datum (averaged from 1960 to 1969).
>
>
> Cheers, Roy.
>
>
> Please note that I partially retired on 01/11/2015. I am now only 
> working 7.5 hours a week and can only guarantee e-mail response on 
> Wednesdays, my day in the office. All vocabulary queries should be 
> sent to enquiries at bodc.ac.uk. Please also use this e-mail if your 
> requirement is urgent.
>
>
>
> ------------------------------------------------------------------------
> *From:* CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of 
> Jonathan Gregory <j.m.gregory at reading.ac.uk>
> *Sent:* 25 May 2017 17:23
> *To:* cf-metadata at cgd.ucar.edu
> *Subject:* Re: [CF-metadata] Standard names for CF trac ticket #143
> Dear Karl
>
> Yes, it is a bit vague. Mean sea level is a commonly used concept, but 
> if you
> wanted to be precise you would use standard names referring to 
> geopotential
> datum or geoid instead, and state which definition was being used in the
> grid mapping. I think vagueness is OK in standard names if there are 
> datasets
> which need to use them.
>
> Actually my "dozen" was an overestimate because not all of them are 
> talking
> about regional sea level. The ones affected are
>
> air_pressure_at_sea_level:Pa
> sea_floor_depth_below_sea_level:m
> sea_surface_height_above_sea_level:m
> surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid:m 
> s-1
> surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid:m 
> s-1
> surface_geostrophic_sea_water_x_velocity_assuming_sea_level_for_geoid:m 
> s-1
> surface_geostrophic_sea_water_y_velocity_assuming_sea_level_for_geoid:m 
> s-1
>
> Best wishes
>
> Jonathan
>
> ----- Forwarded message from Karl Taylor <taylor13 at llnl.gov> -----
>
> > Date: Thu, 25 May 2017 09:05:05 -0700
> > From: Karl Taylor <taylor13 at llnl.gov>
> > To: cf-metadata at cgd.ucar.edu
> > Subject: Re: [CF-metadata] Standard names for CF trac ticket #143
> > User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0)
> >        Gecko/20100101 Thunderbird/52.1.1
> >
> > Dear all,
> >
> > I kinda like the idea of changing "above sea level" to "above mean
> > sea level", but it still remains somewhat vague, since the period
> > over which the mean is computed isn't specified. Or is there some
> > accepted time?  In any case maybe it is o.k. to be vague????
> >
> > best regards,
> > Karl
> >
> >
> > On 5/25/17 7:02 AM, Jonathan Gregory wrote:
> > >Dear Alison and Nan
> > >
> > >Many thanks for doing this, Alison, and apologies that I didn't 
> have time for
> > >it sooner myself. I think this is all fine except for the phrase 
> defining
> > >geopotential datum, which appears in several of them. I would say
> > >
> > >The "geopotential datum" is any estimated surface of constant 
> geopotential used
> > >as a datum i.e. a reference level; for the geoid as a datum, 
> specific standard
> > >names are available.
> > >
> > >The geoid is a geopotential (equipotential) surface, namely the one 
> which
> > >encloses (between itself and the solid Earth below) a volume equal 
> to the
> > >volume of the ocean, often informally described as the surface the 
> ocean would
> > >have if it were at rest. The ellipsoid isn't a geopotential 
> surface. We intro-
> > >duced the term "geopotential datum" in 
> https://cf-trac.llnl.gov/trac/ticket/118 
> <https://cf-trac.llnl.gov/trac/ticket/118>
> > >
> > >Nan commented
> > >
> > >Re: The term 'height_above_sea_level', defined as "Height_above_X" 
> means the
> > >vertical distance above the named surface X. "sea_level" means 
> *mean* sea
> > >level, which is close to the geoid in sea areas.' Someone recording 
> observed
> > >met data on a buoy, looking for the best term for the sensor 
> heights, might
> > >easily choose this instead of 'height', but the height in that case 
> is above
> > >actual sea level; varying over time. If this variable is meant to 
> be 'height
> > >above mean sea level', that should be its name.
> > >
> > >There are about a dozen standard names using sea_level to mean 
> mean_sea_level
> > >consistently, and we don't currently use the phrase mean_sea_level 
> at all. If
> > >people agree, we could rename the existing standard names as Nan 
> suggests for
> > >this new one.
> > >
> > >Best wishes
> > >
> > >Jonathan
> > >
> > >----- Forwarded message from alison.pamment at stfc.ac.uk -----
> > >
> > >>Date: Thu, 18 May 2017 11:37:54 +0000
> > >>From: alison.pamment at stfc.ac.uk
> > >>To: cf-metadata at cgd.ucar.edu
> > >>Subject: [CF-metadata] Standard names for CF trac ticket #143
> > >>
> > >>Dear Jonathan, All,
> > >>
> > >>CF trac ticket #143 (https://cf-trac.llnl.gov/trac/ticket/143) has 
> been agreed and will be included in CF 1.7. A number of new standard 
> names are needed to support the implementation of this ticket. They 
> are names for constants used in the formula_terms attribute of 
> parameterized vertical coordinates.
> > >>
> > >>The following names are proposed.
> > >>
> > >>air_pressure_at_top_of_atmosphere_model (Pa)
> > >>' "Top of atmosphere model" means the upper boundary of the top 
> layer of an atmosphere model.'
> > >>
> > >>altitude_at_top_of_atmosphere_model (m)
> > >>'Altitude is the (geometric) height above the geoid, which is the 
> reference geopotential surface. The geoid is similar to mean sea 
> level. "Top of atmosphere model" means the upper boundary of the top 
> layer of an atmosphere model.'
> > >>
> > >>reference_air_pressure_for_atmosphere_vertical_coordinate (Pa)
> > >>'For models using a dimensionless vertical coordinate, for 
> example, sigma, hybrid sigma-pressure or eta, the values of the 
> vertical coordinate at the model levels are calculated relative to a 
> reference level. "Reference air pressure" is the air pressure at the 
> model reference level. It is a model-dependent constant."
> > >>
> > >>height_above_geopotential_datum_at_top_of_atmosphere_model (m)
> > >>' "Height_above_X" means the vertical distance above the named 
> surface X. The "geopotential datum" is the geopotential reference 
> level (level of zero geopotential) where this is not a specifically 
> named level such as the geoid or a reference ellipsoid. "Top of 
> atmosphere model" means the upper boundary of the top layer of an 
> atmosphere model.'
> > >>
> > >>height_above_geopotential_datum (m)
> > >>'"Height_above_X" means the vertical distance above the named 
> surface X. The "geopotential datum" is the geopotential reference 
> level (level of zero geopotential) where this is not a specifically 
> named level such as the geoid or a reference ellipsoid.'
> > >>
> > >>surface_height_above_geopotential_datum (m)
> > >>' "Height_above_X" means the vertical distance above the named 
> surface X. The surface called "surface" means the lower boundary of 
> the atmosphere. The "geopotential datum" is the geopotential reference 
> level (level of zero geopotential) where this is not a specifically 
> named level such as the geoid or a reference ellipsoid.'
> > >>
> > >>sea_surface_height_above_geopotential_datum (m)
> > >>' "Height_above_X" means the vertical distance above the named 
> surface X. The "geopotential datum" is the geopotential reference 
> level (level of zero geopotential) where this is not a specifically 
> named level such as the geoid or a reference ellipsoid. "Sea surface 
> height" is a time-varying quantity.'
> > >>
> > >>sea_floor_depth_below_geopotential_datum (m)
> > >>' "Depth_below_X" means the vertical distance below the named 
> surface X. The "geopotential datum" is the geopotential reference 
> level (level of zero geopotential) where this is not a specifically 
> named level such as the geoid or a reference ellipsoid.'
> > >>
> > >>sea_floor_depth_below_reference_ellipsoid (m)
> > >>' "Depth_below_X" means the vertical distance below the named 
> surface X. A reference ellipsoid is a regular mathematical figure that 
> approximates the irregular shape of the geoid. A number of reference 
> ellipsoids are defined for use in the field of geodesy.'
> > >>
> > >>height_above_sea_level (m)
> > >>' "Height_above_X" means the vertical distance above the named 
> surface X. "sea_level" means mean sea level, which is close to the 
> geoid in sea areas.'
> > >>
> > >>I have based the definitions on my own reading of ticket #143 and 
> on existing names. I'd welcome comments to improve them.
> > >>
> > >>Best wishes,
> > >>Alison
> > >>
> > >>------
> > >>Alison Pamment Tel: +44 1235 778065
> > >>Centre for Environmental Data Analysis         Email: 
> alison.pamment at stfc.ac.uk
> > >>STFC Rutherford Appleton Laboratory
> > >>R25, 2.22
> > >>Harwell Campus, Didcot, OX11 0QX, U.K.
> > >>
> > >>
> > >>
> > >>_______________________________________________
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