A method of deriving, from distances measured at or computed for the Earth's surface, equivalent distances on the ellipsoid of reference. The distances on the Earth's surface are reduced to the those distances on the geoid which lie on the same normals to the ellipsoid. Angles measured at or computed for the Earth's surface are referred to the geoid without change except for those angles determined at large elevations, when a small correction may be applied. Then, starting from the origin, the distances and angles on the geoid are developed onto the ellipsoid in such a manner that distances and angles remain unchanged throughout the process.