National reference frames and geoid models
It is a well-known fact that the national height systems in Europe refer to different datums and standards and therefore differ at the decimeter or meter level (see the example in the Figure below or Height datum relations for further details). However, although the ETRS89 defines a common standard for three-dimensional coordinates in Europe, a multitude of pan-European realizations (ETRF) as well as national realizations have been established over time. These differ at the sub-dm level. Consequently, the ETRS89 standard must be considered as a datum ensemble.
Transformation parameters from national heights in Europe to EVRF2019 in cm and reference tide gauges (click to enlarge)
The transformation between the global terrestrian reference frames (ITRF) and the corresponding ETRF is done by means of a 14-parameter Helmert transformation. EUREF Technical Note 1 describes the transformation and contains a list of the relevant transformation parameters.
Due to the development of the global space-geodetic observation techniques, the accuracy of the reference frames has generally increased since the first realizations of ETRS89 in the early 1990s. With the ITRF2020, a stability of the geocenter at the level of one millimeter without significant drift has been achieved. On the other hand, most countries still constrain new coordinate solutions to their initial national realizations of ETRS89 (e.g. ETRS89/DREF91 for Germany).
However, in hindsight it becomes clear that the accuracy and stability of the older ETRF until ETRF2000 is only at the sub-decimeter level. If the centimeter matters, the individual national realizations of ETRS89 (relation to specific ETRF and epoch) need to be considered.
This is also particularly relevant for the ellipsoidal height. This is because the translation parameters (and to a little extent also the scale parameter) alter the ellipsoidal height in the transformation, leading to an apparent offset and tilt (static case for 7-parameter Helmert transformation) or even apparent height drift (dynamic case for 14-parameter Helmert transformation with linear velocities of the parameters) between different ETRF and ITRF. In the Figure below the effect is illustrated for the example of the apparent height difference of ETRF2000 with respect to ETRF2020 (an animated plot can be downloaded here). This affects all ETRF versions before ETRF2014, because nowadays the ETRF are related to the corresponding ITRF only by the three rotation parameters, leaving the ellipsoidal height virtually unchanged. This means that ETRF2014 is the first European realization of ETRS89 in which the datum is solely realized by means of a plate rotation model according to the ETRS89 definition.
Apparent offset and tilt of ETRF2000 with respect to ETRF2020 at epoch 2025.0 (click to enlarge)
For this reason, an overview of the national reference frame realizations with focus on the vertical component has been compiled as one activity within the EUREF Working Group "European Unified Height Reference". This includes the national ETRS89 reference frames (ellipsoidal heights) as well as the national vertical reference frames and geoid models.
The information below has been submitted by the national bodies responsible for the geodetic reference since 2022.
Disclaimer: Although it has been checked and compiled with care, no liability is taken that the data are correct and up to date.
Please report any comments and corrections to: quasigeoid@bkg.bund.de
All rights reserved.
Coming soon: Download “EUREF_Overview_Geodetic_Reference_v1.0_2024-xx-xx.pdf
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