Ellipsoid and ITRF

A reference ellipsoid is a mathematical surface defining a geodetic system based on the relative positions of points located on the Earth's surface. The stability of the reference, like any other geodetic datum, depends on the number and geographical distribution of points used to define it, as well as techniques used to locate them. The value of the ellipsoid is that it is a convenient reference for certain spatial techniques, especially in altimetry.

International Terrestrial Reference Frame (ITRF)

According to the recommendations of the IUGG and IAG of 1991, the International Terrestrial Reference System (ITRS) realized with the International Terrestrial Reference Frame (ITRF) should be used for any application that requires an accuracy of better than one meter.


WGS84 versus ITRF

Although WGS84, the GPS reference system, is more popular, this recommendation is still valid because the ITRS is realized from a very dense geographic coverage of several hundred points, compared to only twenty for WGS54, and using different techniques (VLBI, SLR, GPS, Doris), compared to just one for WGS54 (GPS). In practice, the realizations of the two systems are quite similar, but conceptually the ITRS is technology-independent so any new proven technology can be used to improve it. In particular, future developments and the emergence of new positioning systems like Galileo must not be neglected. Another argument (if one were needed) is that the ITRS is maintained and improved by the international community and therefore depends neither on any one agency or country, but on an international scientific association, the International Association of Geodesy (IAG), which provides its scientific quality certification.


To find out more:



  • Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.


Last updated: 12/12/2012