WGS84basis

WGS84basis is a term used informally to describe the WGS84 reference frame’s Cartesian basis in the Earth-centered, Earth-fixed (ECEF) coordinate system. It combines the WGS84 ellipsoid model with a global 3D coordinate system in which positions are expressed as X, Y, and Z, fixed relative to the Earth’s center of mass.

In this basis, the origin is the Earth's center of mass, and the axes are defined as follows: the X-axis passes through the intersection of the equator with the prime meridian (0° longitude), the Y-axis passes through the equator at 90°E, and the Z-axis passes through the north pole. The position of a point is thus given in three-dimensional Cartesian coordinates that remain nearly constant with respect to the Earth’s crust, aside from tectonic motion.

The WGS84 ellipsoid parameters are central to the basis: the semi-major axis a is 6378137.0 meters and the flattening f is 1/298.257223563 (with eccentricity derived from these values). Geodetic coordinates (latitude, longitude, height) can be converted to and from the ECEF coordinates using standard formulas that incorporate these ellipsoid parameters.

WGS84 has multiple realizations and is maintained as a global reference frame by international standards bodies, with updates coordinated through the IERS. It is the basis of GPS positioning and widely used in geodesy, navigation, and GIS. Transformations between WGS84 and other datums (for example, NAD27 or NAD83) commonly use seven-parameter Helmert transformations, reflecting differences in origin, orientation, and scale.

Limitations of the basis include the fact that it is a simplified global model and does not capture local tectonic motions or temporal changes in the Earth's orientation without updates.