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Geodetic aspects of the ETRS89

The subcommission of the International Association of Geodesy (IAG) for the European reference frame (EUREF), in accordance with the first resolution made in Florence in 1990, recommends that European countries adopt the ETRS89 (European Terrestrial Reference System 1989). The EU (European Union), EuroControl (European Organisation for the Safety of Air Navigation), and EuroGraphics (European Association for Computer Graphics), as well as other national and European entities, recommend the transition to this system. The ETRS89 is currently used as a base to develop cartographic work and for all kinds of Earth observation sciences.

ETRS89 is a three-dimensional geodetic reference system, used as standard for highly accurate GPS georeferencing in Europe. It coincides with the ITRS in the ITRF89 1989.0 frame, which is equivalent to the ETRF89 frame, from where the ETRS89 system gets its name, and is based on the GRS80 reference ellipsoid. It is slightly different from the ellipsoid that the WGS84 currently uses. It is a reference system connected to the stable part of the European plate and moves together with the Eurasian Plate.

Even though the ETRS89 European reference system moves constantly like the global ITRSs, the relationship between them is known, and therefore, the coordinates of any georeferenced element can be transferred to an ITRS without losing accuracy.

As an example of the benefited systems, we can mention the real-time positioning system, RTKAT, that the Institut Cartogràfic i Geològic de Catalunya offers users as part of the Integrated Geodetic Positioning System of Catalonia (SPGIC). This system works as a method to increase accuracy of global positioning systems such as GPS, which allows positioning with an accuracy of a few centimetres, consistently with the ETRS89 reference system.

Relationship with ED50

The ED50 geodetic reference system (European Datum 1950) was created during World War II, in which the need to connect the different European geodetic networks internationally was identified. The points that materialised in the ED50 reference system achieved a continental accuracy back then that went from a few metres in Central Europe to more than ten metres in the south. Even though that was already enough to achieve the objectives that encouraged its creation, for the geodetic network of Catalonia it involves accuracy of 10 ppm, which is 10 cm between markers that are 10 km apart.

The need for an ED50 network with higher internal consistency for large-scale work carried out by the ICGC led it to re-observe and adjust the points of the ROI (lower order network) in Catalonia, using GPS techniques. After this adjustment, carried out by the ICGC, an ED50-ICGC framework was obtained from the ROI network, with accuracy of 4 cm. In order to preserve the official nature of the cartographic product, it was guaranteed that all the coordinates obtained were in the error ellipse of the coordinates published by the ICGC. This adjustment of the ROI points by the ICGC was used by the ICGC as a survey marker for its official ED50 products, as it has the accuracy required to produce 1:1,000 cartography for large areas.

The ICGC, therefore, has calculated a transformation to transfer the collected geoinformation from the ED50-ICGC frame to the ETRS89 in Catalonia, which has been made official by the Cartographic Coordination Commission of Catalonia to transform official smaller scale cartography of 1:1,000.

Relationship of ETRS89 and WGS84

In accordance with the IERS/ITRS, the most current WGS84 results (G730 G873 and G1150) and those of the ITRS system (ITRFyy) can be considered identical at 10 cm. So, in order to relate the ETRS89 and WGS84 we shall take into account the equivalence between the ETRS89 and the ITRS.

By way of example, here is an explanation of a way of knowing up to what accuracy you can consider the ETRS89 and WGS94 equivalent in a measurement carried out on 1 January 2011.

We take the coordinates for a point with known speeds such the Bellmunt station BELL. Let's assume that its coordinates match a measurement with absolute GPS carried out on 01/01/2011, using the last frame matching the ETRS the ITRF2005.

We want to observe the difference with the ITRF2000 2000.0 frame, which is the reference recommended by EUREF to express coordinates in ETRS89, which is used by the ICGC. Using EUREF's geodetic calculator, which transforms coordinates from ETRS89 to ITRS, we get the following changes.

MarkerName

Frame

Epoch

X

Y

Z

BELL

ITRF2005

2011.0

4775849.3990

116814.3710

4213018.9680

BELL

ITRF2000

2011.0

4775849.4030

116814.3714

4213018.9478

BELL

ETRF2000

2011.0

4775849.6871

116813.9826

4213018.6512

BELL

ETRF2000

2000.0

4775849.6954

116813.9945

4213018.6563

0.2964

-0.3765

-0.3117

The result differs from the original by around 40 cm. With this result and the equivalence of 10 cm established by the ITRS, we could therefore conclude that the WGS84 and the ETRS89 are equivalent by 42 cm. Taking into account the accuracy of the absolute WGS84 of around 3 m, we can use the result of a GPS receiver directly on the ICGC's reference frames based on the ETRS89 directly.

View ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT

More information about WGS84 on National Geospatial-Intelligence Agency