Modelling of Differences of Height Systems in

Abstract. The introduction of a new national height system, as it is prepared now in Switzerland, arises the problem of an accurate and efficient transformation method between various data sets containing height information. The same problem occurs in the data exchange between neighbouring countries or in the continental unification of vertical reference systems. The combination of GPS deter-mined heights with levelling measurements causes discrepancies between the two data sets even by using a very accurate geoid model. In this paper, the approach we use in Switzerland to model differences of height systems is presented. Especially the treatment of our old distorted level-ling network and its transition to a modern height system, such as normal heights and orthometric heights, is discussed. A further problem is the calculation of the mean gravity along the plumbline, as it is used in the determination of orthometric heights. Instead of an approximation formula, we use a method of using 3D mass models. We present the influence of the different approaches on the orthometric heights