= 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 to the "Map parameters" lists in Examples F.1, F.2, F.3, F.4, F.5, F.6, F.7, and F.8 ===
* ''semi_major_axis'' and either ''semi_minor_axis'' or ''inverse_flattening'' - Used only for ellipsoidal earth approximations.
* ''earth_radius'' - Used only for spherical earth approximations.
* ''longitude_of_prime_meridian'' - Optional (default = 0.0).
=== 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. ||