Prime Meridian

The prime meridian is the reference meridian of zero longitude from which east and west longitude are counted. The International Meridian Conference of 1884 fixed it at the Greenwich transit instrument, and most datums still reference Greenwich, though some historic datums use Paris, Ferro, or other origins, which a datum definition must state. The modern geodetic zero, the IERS Reference Meridian, sits about 102 m east of the Airy transit circle at Greenwich because it is tied to the geocentric frame and the deflection of the vertical. A non-Greenwich prime meridian must be applied as a longitude offset during datum transformation.