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Ticket Resolution Summary Owner Reporter
#118 fixed Add an attribute in Appendix F to identify the geoid or other geopotential datum davidhassell jonathan
Description

Some geophysical quantities, including some vertical coordinates, are defined with reference to the geoid. For example, the standard_name of altitude is the height above the geoid. CF does not have a default geoid, because the convention is used for both model-generated and real-world data, and climate models typically do not use a real-world geoid. When CF is used for real-world data, it may be necessary to desirable to specify which geoid is being used.

This proposal is to define a new attribute geoid_name to be included in the table of Appendix F for the legal attributes of the grid_mapping variable. This attribute will have type S (string) and its description will be "The name of the geoid. Corresponds to an OGC WKT VERT_DATUM name."

Ticket 80, which was agreed a long time ago for inclusion in CF 1.7, also adds some attributes to Appendix F. This proposal is complementary to and consistent with that one.

Jonathan Gregory

#80 fixed Add missing CF parameters to translate Coordinate Reference System properties to/from OGC Well-Known Text format davidhassell etourigny
Description

1. Title

Add missing CF parameters to translate Coordinate Reference System properties to/from OGC Well-Known Text format.

2. Moderator

Jonathan Gregory

3. Requirement

CF-1.6 grid mapping parameters do not allow to fully represent Geographic and Projected Coordinate Reference System (CRS) definitions in Open Geospatial Consortium (OGC) Well-Known Text format.

The following WKT nodes cannot be transformed to/from CF-1.6: PROJCS, GEOGCS, DATUM, SPHEROID, TOWGS84, PRIMEM. In particular the absence of parameters that correspond to DATUM and TOWGS84 parameters can cause datum shifting errors.

Adding new parameters to the CF grid_mapping specification would allow to translate projected and geographic CRS WKT definitions to/from CF grid_mapping definitions without loss of information.

This is a simpler and easier approach than including the full OGC WKT string (as proposed in CF Metadata trac ticket #69) and detecting the potential conflicts between the CF and WKT definitions.

4. Initial Statement of Technical Proposal

The following grid mapping attributes would be added and described in Table F1

  • horizontal_datum_name
  • towgs84 (a double-precision array, with 3, 6 or 7 values)
  • prime_meridian_name
  • reference_ellipsoid_name
  • projected_coordinate_system_name
  • geographic_coordinate_system_name

The rationale for adding the various name parameters is to ensure a 100% compatibility between OGC WKT definitions and CF grid_mapping definitions and also to make the CF definitions more representative. In general, the names should be equal to the ones used in the EPSG database (see <OGP/EPSG>).

Named datums are particularly troublesome, as there is no unique naming scheme in OGC WKT (see <link2> and <CT>). The use of TOWGS84 parameters (inside a DATUM node) allow to approximate datum shifts to the WGS84 datum, but if precise datum shifts are required (using datum shift grids), then it is important that the datums are uniquely identified. It is required to follow the OGR/Cadcorp naming scheme described in the horizontal_datum_name entry for table F1 below (to ensure compatibility with Simple Features and CT specifications).

For reference the names and parameters of PRIMEM and SPHEROID entries from the EPSG database are provided in <link1> (prime_meridian.csv and ellipsoid.csv). DATUM entries, along with their transformed names and preferred TOWGS84 parameters are given in <link1> (horiz_datum.csv).

A similar proposal could be made to account for vertical CRS (VERT_CS in WKT), but this is outside the scope of this proposal.

As an example, the modified “British National Grid” example below would loose information if it were converted to and from CF-1.6:

PROJCS["unnamed",
    GEOGCS["unknown",
        DATUM["unknown",
            SPHEROID["Spheroid",6377563.396,299.3249646000044]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["metre",1]]

The PROJCS, GEOGCS, DATUM and SPHEROID names are lost, but more importantly the TOWGS84 parameters also. The SPHEROID and PRIMEM parameters are recovered due to existing CF grid_mapping parameters, and the PRIMEM name is restored because Greenwich corresponds to longitude_of_prime_meridian = 0.

Changes to section 5.6

<at the end of 1st paragraph>
The translation of CF coordinate variables to/from OGC Well-Known Text (WKT) format is summarized in a few examples below and described in detail in <link1>. 
Example 5.9. Latitude and longitude on the WGS 1984 datum

<CF definition>

OGC WKT :

GEOGCS["WGS 84",
        DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433]]
Example 5.10. British National Grid
<...>
	char crs ;
		crs:grid_mapping_name = "transverse_mercator" ;
		crs:longitude_of_central_meridian = -2. ;
		crs:false_easting = 400000. ;
		crs:false_northing = -100000. ;
		crs:latitude_of_projection_origin = 49. ;
		crs:scale_factor_at_central_meridian = 0.9996012717 ;
		crs:longitude_of_prime_meridian = 0. ;
		crs:semi_major_axis = 6377563.396 ;
		crs:inverse_flattening = 299.324964600004 ;
		crs:projected_coordinate_system_name = "OSGB 1936 / British National Grid" ;
		crs:geographic_coordinate_system_name = "OSGB 1936" ;
		crs:horizontal_datum_name = "OSGB_1936" ;
		crs:reference_ellipsoid_name = "Airy 1830" ;
		crs:prime_meridian_name = "Greenwich" ;
		crs:towgs84 = 375., -111., 431., 0., 0., 0., 0. ;

OGC WKT :

PROJCS["OSGB 1936 / British National Grid",
    GEOGCS["OSGB 1936",
        DATUM["OSGB_1936",
            SPHEROID["Airy 1830",6377563.396,299.3249646000044],
            TOWGS84[375,-111,431,0,0,0,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["metre",1]]

Changes to Table F1

AttributeTypeDescription
horizontal_datum_nameSThe name of the geodetic (horizontal) datum, which corresponds to the procedure used to measure positions on the surface of the Earth. Valid datum names and their associated parameters are given in <link1> (horiz_datum.csv, OGC_DATUM_NAME column) and are obtained by transforming the EPSG name using the following rules (used by OGR and Cadcorp): convert all non alphanumeric characters (including +) to underscores, then strip any leading, trailing or repeating underscores. This is to ensure that named datums can be correctly identified for precise datum transformations (see <link2> for more details). Corresponds to a OGC WKT DATUM node name.
towgs84NThis indicates a list of up to 7 Bursa Wolf transformation parameters., which can be used to approximate a transformation from the horizontal datum to the WGS84 datum. More precise datum transformations can be done with datum shift grids. Represented as a double-precision array, with 3, 6 or 7 values (if there are less than 7 values the remaining are considered to be zero). Corresponds to a OGC WKT TOWGS84 node.
prime_meridian_nameSThe name of the prime meridian associated with the geodetic datum. Valid names are given in <link1> (prime_meridian.csv). Corresponds to a OGC WKT PRIMEM node name.
|reference_ellipsoid_nameSThe name of the reference ellipsoid. Valid names are given in <link1> (ellipsoid.csv). Corresponds to a OGC WKT SPHEROID node name.
geographic_coordinate_system_nameSThe name of the geographic coordinate system (GCS). Corresponds to a OGC WKT GEOGCS node name.
projected_coordinate_system_nameSThe name of the projected coordinate system (PCS). Corresponds to a OGC WKT PROJCS node name.

Changes to References

5. Benefits

By implementing this proposal, we should assure that data producers and software developers can include most of the datum information and other decriptive properties of specific projections.

This proposal is much simpler and easier to manage than the proposal in trac ticket #69, which is entirely optional and has failed to reach consensus so far.

6. Status Quo

Data producers are forced to add custom WKT entries to their data files without any standard, which does not promote interoperability.

#34 fixed Add new grid mappings cf-conventions@… edavis
Description

1. Title

Add new grid mappings to the existing list.

2. Moderator

John Caron

3. Requirement

A number of grid mappings are being used that are not listed in CF. Make additions to the existing list.

This is a follow-up proposal to the proposal in CF Trac ticket #18.

4. Initial Statement of Technical Proposal

Add the following projections:

  • Lambert Cylindrical Equal Area (EPSG 9834 for spherical earth, EPSG 9835)
  • Mercator (EPSG 9804 for one standard parallel, EPSG 9805 for two standard parallels)
  • Orthographic (EPSG 9840)

5. Benefits

Allow existing data using non-CF defined grid mappings to be CF compliant.

6. Status Quo

Data that is otherwise CF compliant using grid mappings that are not supported by CF.

7. Detailed Proposal

CF Wiki page: GridMapNames - currently at version 11

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