wiki:GridMapHorizDatum

Version 3 (modified by edavis, 11 years ago) (diff)

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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.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.

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.