== Geodetic coordinates: what about longitude == 

 The article says that "locations near the surface are described in terms of latitude (φ), longitude (λ) and height (h). The ellipsoid is completely parameterised by the semi-major axis a and the flattening f." 

 However, if only a and f are given, how do we know where the longitude 0 is? If I'm not mistaken, we also have to add a reference meridian.