Dear Alison I think this is all fine. Thank you for your thoroughness. Yes, I agree, we should also refer to ch 5 for the methods to provide a precise definition of a reference ellipsoid.
Best wishes Jonathan ----- Forwarded message from [email protected] ----- > Date: Tue, 4 Jul 2017 14:35:27 +0000 > From: [email protected] > To: [email protected] > Subject: Re: [CF-metadata] Standard names for CF trac ticket #143 > > Dear Jonathan, Roy, Nan and Karl, > > Many thanks for the discussion of these names. > > 1. 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.' > > 2. 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.' > > Jonathan has indicated he is happy with the suggested definitions for these > names and no other comments have been received. Therefore, names (1) and (2) > are accepted for inclusion in the standard name table and will be published > in the July update. > > 3. height_above_sea_level (m) > As Nan and others have pointed out, this name would be better if it were > height_above_mean_sea_level. Indeed in the recently concluded thread > discussing standard names for mean sea level we have agreed to make a similar > change in all existing names where mean_sea_level is the intended meaning > (http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2017/059555.html). In that > thread we also agreed that the definition of mean sea level should be: ' > "Mean sea level" means the time mean of sea surface elevation at a given > location over an arbitrary period sufficient to eliminate the tidal signals.' > Therefore we now have: > height_above_mean_sea_level (m) > ' "Height_above_X" means the vertical distance above the named surface X. > "Mean sea level" means the time mean of sea surface elevation at a given > location over an arbitrary period sufficient to eliminate the tidal signals.' > > This name is accepted for publication in the standard name table and will be > added in the next update at the end of July. > > 4. air_pressure_at_top_of_atmosphere_model (Pa) > ' "Top of atmosphere model" means the upper boundary of the top layer of an > atmosphere model.' > > Jonathan has indicated he is happy with the suggested definition and no other > comments have been received. We have just agreed a new definition for mean > sea level and in the interests of adding at least brief definitions for > hitherto undefined quantities in the standard name table, I think that > air_pressure is another term that would benefit from some attention. Indeed, > the definition of the standard name air_pressure currently reads 'No help > available'. I suggest the following text: > 'Air pressure is the force per unit area which would be exerted when the > moving gas molecules of which the air is composed strike a theoretical > surface of any orientation'. I suggest adding this to the current proposal > and to the 19 existing air_pressure names currently in the standard name > table. Then we'd have: > > air_pressure_at_top_of_atmosphere_model (Pa) > ' "Top of atmosphere model" means the upper boundary of the top layer of an > atmosphere model. Air pressure is the force per unit area which would be > exerted when the moving gas molecules of which the air is composed strike a > theoretical surface of any orientation.' > > What do you think? > > 5. height_above_geopotential_datum_at_top_of_atmosphere_model (m) > 6. height_above_geopotential_datum (m) > 7. surface_height_above_geopotential_datum (m) > 8. sea_surface_height_above_geopotential_datum (m) > 9. sea_floor_depth_below_geopotential_datum (m) > > Jonathan has suggested the definition text for 'geopotential_datum' should be: > '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.' > > I think this looks fine and agree that the definition shouldn't refer to > reference_ellipsoid. Thank you also for reminding me about trac ticket 118 > where the term 'geopotential datum' was first discussed. In that ticket > Jonathan says: > > However, the definitions of standard_names which refer to the geoid or a > > geopotential datum could draw attention to the possibility of precisely > > specifying the reference surface > > by using a grid_mapping attribute. If ticket 143 is agreed, I will make a > > standard_name proposal on the email list in which this point could be > > included. > We are having this discussion because ticket 143 has indeed been agreed, so > I'd suggest that we also need to add the following text to all the defintions > of names 5 - 9: > 'To specify which geoid or geopotential datum is being used as a reference > level, a grid_mapping variable should be attached to the data variable as > described in Chapter 5.6 of the CF Convention.' > The names would then be as follows: > > 5. 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 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. To specify which geoid or geopotential datum is > being used as a reference level, a grid_mapping variable should be attached > to the data variable as described in Chapter 5.6 of the CF Convention. "Top > of atmosphere model" means the upper boundary of the top layer of an > atmosphere model.' > > 6. height_above_geopotential_datum (m) > ' "Height_above_X" means the vertical distance above the named surface X. 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. To specify which geoid or geopotential datum is > being used as a reference level, a grid_mapping variable should be attached > to the data variable as described in Chapter 5.6 of the CF Convention.' > > 7. 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 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. To specify which geoid or geopotential datum is > being used as a reference level, a grid_mapping variable should be attached > to the data variable as described in Chapter 5.6 of the CF Convention.' > > 8. sea_surface_height_above_geopotential_datum (m) > ' "Height_above_X" means the vertical distance above the named surface X. > "Sea surface height" is a time-varying quantity. 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. To specify which geoid or geopotential datum is being used as a > reference level, a grid_mapping variable should be attached to the data > variable as described in Chapter 5.6 of the CF Convention.' > > 9. sea_floor_depth_below_geopotential_datum (m) > ' "Depth_below_X" means the vertical distance below the named surface X. 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. To specify which geoid or geopotential datum is > being used as a reference level, a grid_mapping variable should be attached > to the data variable as described in Chapter 5.6 of the CF Convention.' > > Are these okay? > > 10. 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.' > > Presumably we should also add some text about using grid_mapping to specify > which reference ellipsoid is being used to the definition of this name? So > we'd have: > ' "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. To specify which reference ellipsoid is > being used, a grid_mapping variable should be attached to the data variable > as described in Chapter 5.6 of the CF Convention.' > > Okay? > > I assume we should also now update the definitions of all the existing ten > geoid names and five reference_ellipsoid names to include the text about > using grid_mapping. Do others agree? > > Best wishes, > Alison > > ------ > Alison Pamment Tel: +44 > 1235 778065 > Centre for Environmental Data Analysis Email: > [email protected] > STFC Rutherford Appleton Laboratory > R25, 2.22 > Harwell Campus, Didcot, OX11 0QX, U.K. > > > _______________________________________________ > 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
