IERS Reference Meridian
The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the transit circle or about 102 metres (335 ft) at the latitude of the Royal Observatory, Greenwich. It is also the reference meridian of the Global Positioning System (GPS) operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF).
The 5.3-arcsecond shift is a legacy of the first satellite navigation system, the Doppler based TRANSIT system. TRANSIT was developed by the Applied Physics Laboratory of Johns Hopkins University. Its lab is located in Howard County, Maryland, which was the location of TRANSIT's first ground station. The station's surveyed coordinates in the North American Datum 1927 (NAD27) — a non-Earth centered ellipsoid — became its coordinates in an Earth-centered ellipsoid, such as the World Geodetic System. This shifted the coordinates of any other location on an Earth-centered ellipsoid, especially those far away.
When the antenna of a TRANSIT ground station was mounted directly above Airy's transit circle in June 1969, its longitude on an Earth-centered ellipsoid was 5.64 arcseconds west of TRANSIT's reference meridian. Several small additional longitude shifts were created by further improvement in gravitational models such as the Earth Geopotential Model 1996 (EGM96), a dramatic increase in the number of ground stations from only four to over 500, and the use of time-based GPS.
The  Tectonic plates slowly move over the surface of the Earth, so most countries have adopted for their maps an IRM version fixed relative to their own tectonic plate as it existed at the beginning of a specific year. Examples include the North American Datum 1983 (NAD83), the European Terrestrial Reference Frame 1989 (ETRF89), and the Geocentric Datum of Australia 1994 (GDA94). Versions fixed to a tectonic plate differ from the global version by at most a few centimetres.
However, the IRM is not fixed to any point on Earth. Instead, all points on the European portion of the Eurasian plate, including the Royal Observatory, are slowly moving northeast about 2.5 cm per year relative to it. Thus this IRM is the weighted average (in the least squares sense) of the reference meridians of the hundreds of ground stations contributing to the IERS network. The network includes GPS stations, Satellite Laser Ranging (SLR) stations, Lunar Laser Ranging (LLR) stations, and the highly accurate Very Long Baseline Interferometry (VLBI) stations. All stations' coordinates are adjusted annually to remove net rotation relative to the major tectonic plates. If Earth had only two hemispherical plates moving relative to each other around any axis which intersects their centres or their junction, then the longitudes (around any other rotation axis) of any two, diametrically opposite, stations must move in opposite directions by the same amount.
Universal Time is notionally based on the WGS84 meridian. Because of changes in the rate of Earth's rotation, standard international time UTC can differ from the mean observed solar time at noon on the prime meridian by up to 0.9 second. Leap seconds are inserted periodically to keep UTC close to Earth's angular position relative to the Sun; see mean solar time.
List of places
Country, territory or sea Notes Arctic Ocean Greenland Sea Norwegian Sea North Sea United Kingdom From Tunstall in East Riding to Peacehaven English Channel France From Villers-sur-Mer to Gavarnie Spain From Cilindro de Marboré to Castellón de la Plana Mediterranean Sea Gulf of Valencia Spain From El Verger to Calpe Mediterranean Sea Algeria From Stidia to Algeria-Mali border near Bordj Mokhtar Mali Burkina Faso Togo For about 600 m Ghana For about 16 km Togo For about 39 km Ghana From the Togo-Ghana border near Bunkpurugu to Tema
Passing through Lake Volta at
Atlantic Ocean Passing through the Equator at Southern Ocean Antarctica Queen Maud Land, claimed by Norway
- History of the Prime Meridian – Past and Present
- IRM on grounds of Royal Observatory from Google Earth Accessed 30 March 2012
- The astronomic latitude of the Royal Observatory is 51°28'38"N whereas its latitude on the European Terrestrial Reference Frame (1989) datum is 51°28'40.1247"N.
- PDF (4.89 MB) Section 2.4.4.
- WGS 84 Implementation Manual page i, 1998
- PDF (419 KB)