29 | | * ''semi_major_axis'' and either ''semi_minor_axis'' or ''inverse_flattening'' - Used only for ellipsoidal earth approximations. |

30 | | * ''earth_radius'' - Used only for spherical earth approximations. |

31 | | * ''longitude_of_prime_meridian'' - Optional (default = 0.0). |

| 29 | 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. |

| 30 | |

| 31 | 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. |

| 32 | |

| 33 | More complete definitions of the horizontal datum attributes are given in Table F.1. |