6378137

6378137 refers to the equatorial radius of the WGS84 ellipsoid, expressed in meters. It is the semi-major axis a of the WGS84 reference ellipsoid, defined as exactly 6,378,137.0 meters.

In the WGS84 model, the semi-major axis a is 6,378,137.0 m and the inverse flattening is 298.257223563, yielding a polar radius b of about 6,356,752.314245 m. The flattening f is approximately 0.003352810664747. These values describe Earth's shape more accurately than a perfect sphere, acknowledging that Earth is slightly oblate.

6378137 m is widely used in geodesy, mapping, and GIS. It underpins many geographic calculations and coordinate transformations, including the Web Mercator projection (EPSG:3857), which adopts a spherical model with radius 6378137 m for simplicity. While this radius is convenient for software and cartographic purposes, it is not the mean Earth radius; the mean (authalic) radius is about 6,371 km, and the polar radius is smaller than the equatorial radius. The distinction between the equatorial radius and the mean radius is important for precise geodetic work, but 6378137 m remains a standard constant in many widely used systems.