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Ticket Resolution Summary Owner Reporter
#138 fixed Clarification of false_easting / false_northing davidhassell heiko.klein
Description

The explanation of false_easting/northing has left us often puzzled weather false_easting should be added to the existing x-axis values, or subtracted. To clarify this, I suggest the following modification to Table F.1:

false_easting

The value applied to all abscissa values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate. The formula to convert from a projection-axis value without false_easting (x0) to a projection-axis value with false_easting is (xf) is:
x0 = xf - false_easting

false_northing

The value applied to all ordinate values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_y_coordinate. The formula to convert from a projection-axis value without false_northing (y0) to a projection-axis value with false_northing is (yf) is then:
y0 = yf - false_northing

#139 fixed Update to conformance document: standard_names of area_type & region davidhassell ros
Description

Section 3.3. of the CF convention states:

"Some standard names (e.g. region and area_type) are used to indicate quantities which are permitted to take only certain standard values. This is indicated in the definition of the quantity in the standard name table, accompanied by a list or a link to a list of the permitted values."

but there is no corresponding statement in the conformance document. I therefore propose to add the following to section 3.3 of the CF Conformance document:

"If a variable has a standard_name of region or area_type, it must have value(s) from the permitted list."

#143 fixed Supplement the definitions of dimensionless vertical coordinates davidhassell jonathan
Description

1 Title

Supplement the definitions of dimensionless vertical coordinates

2 Moderator

Rich Signell

3 Purposes

  1. Rename dimensionless coordinates as parameterized coordinates because one of them (hybrid height) is not dimensionless, and dimensionlessness is not their defining characteristic.
  1. Define a new attribute to supply the standard_name of the dimensional coordinates computed by the formula. This may depend on the standard_name of one or more of the terms. Spell out the possibilities for each case in Appendix D and correct some errors.
  1. Point out that some vertical coordinates refer to a vertical datum surface which is described by grid_mapping information.

4 Status quo and benefits

At the moment it is difficult for a data user to know exactly what the computed vertical coordinate is. For example, does the ocean sigma coordinate give you a height with respect to a geopotential surface or a reference ellipsoid, and precisely what is that datum e.g. NAVD88? These changes are intended to make it easier.

5 Detailed proposal

Insert a new section heading "4.3.3 Parameterized Vertical Coordinate" after the sentence "The units attribute is not required ..." in section 4.3.2. Thus, the remainder of that section, about formula terms etc., will become a new section under the new heading.

Amend the existing first paragraph of this new section from the existing text:

For dimensionless vertical coordinates we extend the COARDS standard by making use of the standard_name attribute to associate a coordinate with its definition from Appendix D, Dimensionless Vertical Coordinates. The definition provides a mapping between the dimensionless coordinate values and dimensional values that can positively and uniquely indicate the location of the data. A new attribute, formula_terms, is used to associate terms in the definitions with variables in a netCDF file. To maintain backwards compatibility with COARDS the use of these attributes is not required, but is strongly recommended.

to read as follows:

In some cases dimensional vertical coordinates are parameterized as a function of horizontal as well as vertical location and therefore cannot be stored in the one-dimensional vertical coordinate variable, which in most of these cases is dimensionless. The standard_name of the vertical coordinate variable can be used to find the definition of the associated dimensional vertical coordinate in Appendix D, Parameterized Vertical Coordinates. The definition provides a mapping between the parameterized coordinate values and dimensional values that can positively and uniquely indicate the location of the data. The formula_terms attribute can be used to associate terms in the definitions with variables in a netCDF file, and the computed_standard_name attribute can be used to supply the standard_name of the dimensional vertical coordinate values computed according to the definition. To maintain backwards compatibility with COARDS the use of these attributes is not required, but is strongly recommended. Some of the definitions may be supplemented with information stored in the grid_mapping variable about the datum used as a vertical reference (e.g. geoid, other geopotential datum or reference ellipsoid; see Section 5.6, Horizontal Coordinate Reference Systems, Grid Mappings, and Projections and Appendix F, Grid Mappings).

In this section, in the sentence after Example 4.3, change "Appendix D, Dimensionless Vertical Coordinates" to "Appendix D, Parameterized Vertical Coordinates".

In section 5.6, replace the existing paragraph

When the coordinate variables for a horizontal grid are longitude and latitude, a grid mapping variable with grid_mapping_name of latitude_longitude may be used to specify the ellipsoid and prime meridian.

with

The grid_mapping variable may identify datums (such as the reference ellipsoid, the geoid or the prime meridian) for horizontal or vertical coordinates. Therefore a grid mapping variable may be needed when the coordinate variables for a horizontal grid are longitude and latitude. The grid_mapping_name of latitude_longitude should be used in this case.

In Appendix A, between the entries for compress and Conventions, insert an entry for computed_standard_name, Type S, Use C, Link "Section 4.3.3, Parameterized Vertical Coordinate", Description "Indicates the standard name, from the standard name table, of the parameterized dimensional vertical coordinate values computed according to the formula in the definition."

In Appendix A, change the Link for formula_terms to "Section 4.3.3, Parameterized Vertical Coordinate".

Rename Appendix D as "Parameterized vertical coordinates".

Change the first sentence of Appendix D from "The definitions given here allow an application to compute dimensional coordinate values from the dimensionless ones and associated variables." to "The definitions given here allow an application to compute dimensional coordinate values from the parameterized (usually dimensionless) ones and associated variables."

In the first paragraph of Appendix D, change "where term is a keyword" to "where term is a case-insensitive keyword".

Append the following as a third paragraph in the preamble of Appendix D:

The variables containing the terms may optionally have standard_name attributes, with values as indicated in this Appendix. The standard_name of the dimensional coordinate values which are computed by the formula may optionally be specified by the computed_standard_name attribute of the vertical coordinate variable, as indicated in this Appendix. A computed_standard_name is uniquely implied by the formula in some cases, while in others it depends on the standard_name of one or more of the terms, with which it must be consistent.

In the following, it will be noticed that some formula terms do not have defined standard names at the moment. Of course, names could be defined, but it is not essential because their purpose is identifiable from the formula_terms attribute. However, standard_names have been proposed below for constants in the formula which don't depend on level e.g. reference air pressure.

Append the following paragraph to the definition of "Atmosphere natural log pressure coordinate":

The standard_name of p0 is reference_air_pressure_for_atmosphere_vertical_coordinate. The computed_standard_name is air_pressure.

Append the following paragraph to the definition of "Atmosphere sigma coordinate":

The standard_name of ptop is air_pressure_at_top_of_atmosphere_model, and of ps is surface_air_pressure. The computed_standard_name is air_pressure.

Append the following paragraph to the definition of "Atmosphere hybrid sigma pressure coordinate":

The standard_name of p0 is reference_air_pressure_for_atmosphere_vertical_coordinate, and of ps is surface_air_pressure. The computed_standard_name is air_pressure. No standard_name has been defined for a, b or ap.

In the definition of "Atmosphere hybrid height coordinate", change "z(n,k,j,i) is the height above the geoid (approximately mean sea level) at gridpoint (k,j,i) and time (n) , orog(n,j,i) is the height of the surface above the geoid" to "z(n,k,j,i) is the height above the datum (e.g. the geoid, which is approximately mean sea level) at gridpoint (k,j,i) and time (n) , orog(n,j,i) is the height of the surface above the datum", and replace the final paragraph "There is no dimensionless ..." with the following.

The standard_name of orog may be surface_altitude (i.e. above the geoid) or surface_height_above_geopotential_datum. The computed_standard_name is altitude or height_above_geopotential_datum in these cases respectively. No standard_name has been defined for b. There is no dimensionless coordinate because a, which has the standard_name of atmosphere_hybrid_height_coordinate, is the best choice for a level-dependent but geographically constant coordinate.

In the definition of "Atmosphere smooth level vertical (SLEVE) coordinate", change "z(n,k,j,i) is the height above the geoid (approximately mean sea level) at gridpoint (k,j,i) and time (n), ztop is the height of the top of the model" to "z(n,k,j,i) is the height above the datum (e.g. the geoid, which is approximately mean sea level) at gridpoint (k,j,i) and time (n), ztop is the height of the top of the model above the datum", change "the large and small parts of the topography" to "the large-scale and small-scale components of the topography, and a, b1 and b2 are all functions of the dimensionless SLEVE coordinate" (for the sake of clarification), delete the sentence "The hybrid height coordinate for level k is defined as a(k)*ztop" (which appears to be in the wrong section) and append the following paragraph:

The standard_name of ztop may be altitude_at_top_of_atmosphere_model (i.e. above the geoid) or height_above_geopotential_datum_at_top_of_atmosphere_model. The computed_standard_name is altitude or height_above_geopotential_datum in these cases respectively. No standard_name has been defined for b1, zsurf1, b2 or zsurf2.

In the definition of "Ocean sigma coordinate", replace "z(n,k,j,i) is height, positive upwards, relative to ocean datum (e.g. mean sea level) at gridpoint (n,k,j,i), eta(n,j,i) is the height of the ocean surface, positive upwards, relative to ocean datum at gridpoint (n,j,i)" with "z(n,k,j,i) is height (positive upwards) relative to the datum (e.g. mean sea level) at gridpoint (n,k,j,i), eta(n,j,i) is the height of the sea surface (positive upwards) relative to the datum at gridpoint (n,j,i)", replace "depth(j,i) is the distance from ocean datum to sea floor (positive value) at horizontal gridpoint (j,i)" with "depth(j,i) is the distance (a positive value) from the datum to the sea floor at horizontal gridpoint (j,i)", and append the following paragraph:

The standard_names for eta and depth and the computed_standard_name must be one of the consistent sets shown in Table D.1.

In the definition of "Ocean s-coordinate", make the same replacements as for "Ocean sigma coordinate", and append the following paragraph:

The standard_names for eta and depth and the computed_standard_name must be one of the consistent sets shown in Table D.1. No standard_name has been defined for a, b or depth_c.

In the definition of "Ocean sigma over z coordinate", make the same replacements as for "Ocean sigma coordinate", after "Above depth depth_c there are nsigma layers" insert "and below this depth the levels are surfaces of constant height zlev (positive upwards) relative to the datum" (this definition is omitted in the current text), and append the following paragraph:

The standard_names for eta, depth, zlev and the computed_standard_name must be one of the consistent sets shown in Table D.1. No standard_name has been defined for depth_c or nsigma.

In the definition of "Ocean double sigma coordinate", make the same replacements as for "Ocean sigma coordinate" regarding z and depth (there is no eta in this case), and append the following paragraph:

The standard_name for depth and the computed_standard_name must be one of the consistent sets shown in Table D.1. No standard_name has been defined for z1, z2, a, href or k_c.

At the end of Appendix D, insert the following table:

Table D.1. Consistent sets of values for standard_name and computed_standard_name for the formula terms indicated.

computed_standard_name of altitude with standard_name of altitude for zlev, sea_surface_height_above_geoid for eta and sea_floor_depth_below_geoid for depth.
computed_standard_name of height_above_geopotential_datum with standard_name of height_above_geopotential_datum for zlev, sea_surface_height_above_geopotential_datum for eta and sea_floor_depth_below_geopotential_datum for depth.
computed_standard_name of height_above_reference_ellipsoid with standard_name of height_above_reference_ellipsoid for zlev, sea_surface_height_above_reference_ellipsoid for eta and sea_floor_depth_below_reference_ellipsoid for depth.
computed_standard_name of height_above_sea_level with standard_name of height_above_sea_level for zlev, sea_surface_height_above_sea_level for eta and sea_floor_depth_below_sea_level for depth.

In the conformance document, rename "4.3.2 Dimensionless Vertical Coordinates" to "4.3.3 Parameterized Vertical Coordinate", and append these new requirements:

  • Where indicated by the appropriate definition in Appendix D, the standard_name attributes of variables named by the formula_terms attribute must be consistent with the standard_name of the coordinate variable it is attached to, according to the appropriate definition in Appendix D.
  • The computed_standard_name attribute is only allowed on a coordinate variable which has a formula_terms attribute.
  • The computed_standard_name attribute is a string whose value must be consistent with the standard_name of the coordinate variable it is attached to, and in some cases also with the standard_name attributes of variables named by the formula_terms attribute, according to the appropriate definition in Appendix D.

If this ticket is agreed, the following standard_names will be proposed:

air_pressure_at_top_of_atmosphere_model
altitude_at_top_of_atmosphere_model
reference_air_pressure_for_atmosphere_vertical_coordinate
height_above_geopotential_datum_at_top_of_atmosphere_model
height_above_geopotential_datum
surface_height_above_geopotential_datum
sea_surface_height_above_geopotential_datum
sea_floor_depth_below_geopotential_datum
sea_floor_depth_below_reference_ellipsoid
height_above_sea_level

Jonathan

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