ABSTRACT
The observations for the Lotschberg tunnel in Switzerland were also corrected for plumb line deviations since the influence was of the same dimension as in the Simplon tunnel. The stations, elevations and radii shown on the alignment and longitudinal section refer to the invert of the tunnel. The measurement implies the orientation of at least one side of the tunnel control survey at the surface, in order to calculate the bearing of the centre line in the same system. The connection with the control survey could be established by means of Weisbach triangles. The main source of errors is the centring of the wires. A precision of less than some tenths of a millimetre is hardly attainable in practice especially in deep shafts. In practice the terminals are often destroyed by the construction works and at the same time a number of directions to triangulation stations.
