= Grid Mapping: Adding Horizontal Datum =
== Title ==
Add horizontal datum attributes to the existing set of grid mapping attributes.
== Requirement ==
Additional CF grid mapping attributes are needed before a projection can be fully specified. The existing CF grid mapping attributes can specify a projection method and projection coordinates but not the associated geographic coordinates.
This is a follow-up proposal to the proposal in CF Trac ticket #18.
== Summary of Change ==
Add grid mapping attributes to describe the geographic CRS upon which the projection is based. This will include attributes that describe the horizontal datum (ellipsoid/sphere) and prime meridian along with reasonable default values.
== Benefit ==
These additions will allow more complete specification of projections. This will allow better mapping between CF and OGC CRS.
== Status Quo ==
Incomplete specification of projection information.
== Initial Statement of Technical Proposal ==
=== Add the following paragraphs to Appendix F before Example F.1 ===
Attributes for specifying the horizontal datum are common to all grid mappings. The horizontal datum specifies a prime meridian and an ellipsoid or sphere used to approximate the shape of the earth. The prime meridian, which defines the origin from which longitude is measured, should be specified by the value of the ''longitude_of_prime_meridian'' attribute. The ''longitude_of_prime_meridian'' attribute is optional, its default value is 0.0.
When an ellipsoid is used to approximate the shape of the earth, it should be specified with the ''semi_major_axis'' attribute and either the ''semi_minor_axis'' attribute or the ''inverse_flattening'' attribute. When a sphere is used, it should be specified with the ''earth_radius'' attribute. If only the ''semi_major_axis'' is specified, a client should use that value as the radius of a spherical earth. If none of the four attributes are specified, a client should assume a standard spherical or ellipsoidal earth. For instance, the "GRS 1980 Authalic Sphere" (EPSG 7048) defines the earth's radius as 6371007 meters and the "GRS 1980" ellipsoid (EPSG 7019) defines the semi-major axis as 6378137 meters and the inverse flattening as 29.257222101.
More complete definitions of the horizontal datum attributes are given in Table F.1.
=== Add the following attributes to Table F.1 ===
|| '''Attribute''' || '''Description''' ||
|| earth_radius || Used to specify the radius, in metres, of the spherical figure used to approximate the shape of the Earth. This attribute should be specified for those projections in which the X-Y cartesian coordinates have been derived using a spherical Earth approximation. If the cartesian coordinates were derived using an ellipsoid to approximate the shape of the Earth, then this attribute should not be defined. Example: "6371007” , which is the radius of the GRS 1980 Authalic Sphere. ||
|| inverse_flattening || Used to specify the inverse flattening (1/f) of the ellipsoidal figure associated with the geodetic datum and used to approximate the shape of the Earth. The flattening (f) of the ellipsoid is related to the semi-major and semi-minor axes by the formula f = (a-b)/a. In the case of a spherical Earth this attribute should be omitted (see the ''earth_radius'' attribute). Example: 298.257222101 for the GRS 1980 ellipsoid. (Note: By convention the dimensions of an ellipsoid are specified using either the semi-major and semi-minor axis lengths, or the semi-major axis length and the inverse flattening. If all three attributes are specified then the supplied values must be consistent with the aforementioned formula.) ||
|| longitude_of_prime_meridian || Specifies the longitude, with respect to Greenwich, of the prime meridian associated with the geodetic datum. The prime meridian defines the origin from which longitude values are determined. Domain: -180.0 <= longitude_of_prime_meridian < 180.0 decimal degrees. Default = 0.0 ||
|| semi_major_axis || Specifies the length, in metres, of the semi-major axis of the ellipsoidal figure associated with the geodetic datum and used to approximate the shape of the Earth. Commonly denoted using the symbol “a”. In the case of a spherical Earth approximation this attribute should be omitted (see the ''earth_radius'' attribute). See also the ''inverse_flattening'' attribute. ||
|| semi_minor_axis || Specifies the length, in metres, of the semi-minor axis of the ellipsoidal figure associated with the geodetic datum and used to approximate the shape of the Earth. Commonly denoted using the symbol “b”. In the case of a spherical Earth approximation this attribute should be omitted (see the ''earth_radius'' attribute). See also the ''inverse_flattening'' attribute. ||