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 enquir...@bodc.ac.uk. Please also use this e-mail if your
requirement is urgent.
------------------------------------------------------------------------
*From:* CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of
Jonathan Gregory <j.m.greg...@reading.ac.uk>
*Sent:* 25 May 2017 17:23
*To:* cf-metadata@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 <taylo...@llnl.gov> -----
> Date: Thu, 25 May 2017 09:05:05 -0700
> From: Karl Taylor <taylo...@llnl.gov>
> To: cf-metadata@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.pamm...@stfc.ac.uk -----
> >
> >>Date: Thu, 18 May 2017 11:37:54 +0000
> >>From: alison.pamm...@stfc.ac.uk
> >>To: cf-metadata@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.pamm...@stfc.ac.uk
> >>STFC Rutherford Appleton Laboratory
> >>R25, 2.22
> >>Harwell Campus, Didcot, OX11 0QX, U.K.
> >>
> >>
> >>
> >>_______________________________________________
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