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Horizontal Reference Systems

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Horizontal Reference Systems


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.

    Examples: World Geodetic System 1984 (WGS-84), International/European Terrestrial Reference System (ITRS/ETRS) and national reference systems.

1.2 Reference Frame
A reference frame is a realisation of a reference system through a consistent set of 3-dimensional (3D) station co-ordinates, taking into account the continental drifts.
Examples: European Terrestrial Reference Frame (ETRF) 89 (valid as of January 1st 1989), ETRF90, ETRF91, etc.

1.3 WGS-84
WGS-84 defines a global terrestrial reference system (geodetic datum) and a geocentric reference ellipsoid9. It was developed by the United States Department of Defence, together with scientists of other countries and institutions. WGS-84 is currently the reference system ICAO requires for georeferencing aeronautical information.

1.4 The International Terrestrial Reference System
As was seen with WGS-84, the ITRS is a global terrestrial reference system. The ITRS is maintained by the International Earth Rotation and Reference Systems Service (IERS) and the realisation of the ITRS is the International Terrestrial Reference Frame (ITRF).

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)
ETRS89 is a reference system based on the ITRS. Like ITRS, it uses the Geodetic Reference System 1980 (GRS80) reference ellipsoid which is slightly different to the WGS-84 reference ellipsoid.

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
The theoretical principles of both the WGS-84 and ETRS89 systems are the same. For WGS-84, the position of the reference ellipsoid was initially calculated on the basis of available data and modelled as a best fit for the whole world but with limited precision (initially 1-2 metres). ITRS2000 is the latest instantiation of WGS-84. ETRS89 was identical to ITRS at the 1989 epoch. ETRS89 is only used in Europe but the relationship between ITRS and ETRS is well known (and transformation parameters are available for the various epochs). The reference network for WGS-84 consists of only 13 stations around the world, whereas the European Reference Network (ERN) consists of over 100 stations within Europe. In practical terms, this means that Global Positioning System (GPS) surveys within Europe will need to be based on ETRS89, and converted to ITRS, as necessary.

1.7 Universal Transverse Mercator (UTM)
ETRS89 describes space in a 3D ellipsoidal co-ordinate system. To obtain 2- dimensional (2D) planar co-ordinates that are used for a wide range of applications, co-ordinates are transformed using a map projection. Co-ordinates in a planar system, such as Universal Transverse Mercator (UTM), are much easier to use and understand, and objects can be published at different scales, and on digital or analogue devices. A variety of different map projections exist, each being optimised for a certain application or region (often used in combination with a local, best fitting ellipsoid). At a global level, UTM has become very popular in recent years and many countries have started substituting the local map projection with UTM. UTM, as an isogonic projection, is suitable for aviation charting. Another advantage of UTM over many local map projections is the simplicity of the projection: x/y co-ordinates in UTM can be easily projected to ETRS89 ellipsoidal co-ordinates, and vice versa, because they are both based on the same reference ellipsoid.

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
Although surveying using triangulation was considered a very accurate technique in the early 20th century, “old” national networks contain scalar and angular errors and inconsistencies. These torsions are mainly due to blunders in the measurement of reference distances (base measurements). The torsions can easily reach several metres between the most remote areas of a country.

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.

Deviation Between Old and New Horizontal Reference Frames

Source: Eurocontrol Terrain and Obstacle Data Manual

See also:
Horizontal Reference Systems
Vertical Reference Systems
Temporal Reference Systems

The presented material is for training purpose only!