Three different approaches for the unification of height reference frames have been used to compute datum offsets between national vertical reference frames in Europe: the oceanographic approach, the spirit leveling approach and the gravity field approach. All three methods are discussed and advantages and drawbacks are evaluated. A set of tide gauge locations is used to compute height datum offsets between national height reference frames in Europe based on all three approaches. The results agree on a level of 5–10 cm. In summary, the gravity field approach is the most flexible approach. The ESA satellite gravity mission GOCE ensures a uniform global level in the range of 1–2 cm. Hence, the gravity field approach is a suitable approach for future realizations of the European Vertical Reference Frame as well as for the establishment of a World Height System.

A suitable representation of the regional gravity eld is used to estimate relative offsets between national height system realizations in Europe. The method used is based on a gravimetric approach and bene ts from the signi cant improvements in the determination of the global gravity eld by the recent satellite gravity missions the Gravity Recovery and Climate Experiment (GRACE) and the Gravity eld and steady-state Ocean Circulation Explorerr (GOCE). The potential of thesemissions for the uni cation of height reference frames is analyzed in terms of accuracy and spatial resolution. The results of the gravimetric approach are compared to the independent results of the geodetic leveling approach. Advantages and drawbacks of both methods are discussed.

Since the last UELN report in 2002 the United European Levelling Network (UELN) could be extended to South East Europe due to the integration of the Bulgarian network. The first order levelling network of Bulgaria consists of 36 nodal points and 57 measurements between them. The connection to the UELN is realized by 6 levelling lines between the networks of Romania and Bulgaria. The paper provides information about the Bulgarian levelling network as well as some results of the latest UELN adjustment (UELN-95/17).

Since the last UELN report in 2004 two new data sets were included in the UELN database. In May 2004 the Danish network block was substituted by a new version of the same epoch (1980-1995). In December 2004 the 5th Primary Levelling of the Netherlands was handed over. The data of the new levelling networks of Finland, Norway and Sweden are expected for the very next time. The Information System for European Coordinate Reference Systems (CRS - EU) was refined. In this paper the available information about the different height systems in Europe will be introduced. 1. Status of the UELN Data Base 1.1. Replacement of the Danish Network