1.1 Reference System
A reference system provides a definition of a co-ordinate system in terms of the position of an origin in space, the orientation of an orthogonal set of Cartesian axes, and a scale. A terrestrial reference system defines a spatial reference system in which positions of points anchored on the Earth’s solid surface have co-ordinates.
1.2 Reference Frame
1.4 The International Terrestrial Reference System
Plate tectonic movement has been incorporated in the ITRS co-ordinate system using the results of recent measurements and a global geophysical model. Thus, it is a model with changing co-ordinates due to the movement of the tectonic plates on which the ground stations are located. However, ITRS provides the fundamental position of the Earth to within 10cm and the orientation of the axes to correspondingly high accuracies. Since 1988, the IERS has defined the mean spin axis, the IERS Reference Pole, the zero meridian and the IERS Reference Meridian.
Whilst WGS-84 is not a dynamic model, the maintenance of a datum at a higher level of accuracy as for the ITRS requires constant monitoring of the rotation of the Earth, the motion of the pole and the movement of the plates of the crust of the Earth, on which the ground stations are located. Whilst WGS-84 is defined by only 13 reference stations globally, ITRS is defined by a network of many reference stations. The continuous measurement from these stations is used to determine the dynamic variables of the ITRS.
1.5 European Terrestrial Reference System 1989 (ETRS89)
For its realisation (ETRF89), the positions of the ITRS stations in and around Europe, at the beginning of 1989, were used as a reference. Only stations on the stable part of the Eurasian plate were used as these are considered to be consistent. Due to the continental drift of the Eurasian plate, ITRF and ETRF89 co-ordinates differed by about 25cm in the year 2000, a difference which is increasing by about 2.5cm per year.
1.6 Relationship between WGS-84, ITRS and ETRS89
1.7 Universal Transverse Mercator (UTM)
Recent Developments in Co-ordinate Reference Frames
2.1 Reference Frame for Europe
Since national reference frames generally use locally adjusted ellipsoids which are a best fit for the earth surface of a country (such as Bessel 1841), they are not suitable for projects involving different countries. In this context, a continental system such as UTM/ETRS89, as a reference frame for Europe, is preferred because it is a general, best fit for a large area. Such a system simplifies the process of exchanging data between different countries, integrating data into global systems or using positioning services from permanent GPS networks.
2.2 National Reference Frames
Thus, GPS measurements are only consistent with those existing co-ordinates in the “old” national co-ordinate system which are at a very close distance to their reference station. The reference station must be established on a point whose co-ordinates are known in the old national co-ordinate system.
For this reason, many countries started the development of new national reference frames based on GPS measurements. In Europe, these frames are linked to ETRS89 but adjusted for local purposes. Therefore, they are often based on a different ellipsoid to that which is used by ETRS89 (GRS80), such as Bessel or Clarke.
Source: Eurocontrol Terrain and Obstacle Data Manual