The Brenner Base Tunnel - a new link through the Alps

20 During the construction of the tunnel the principle of the interlaced polygon is applied as a surveying technique by continuously measuring angles and distances up into the tunnel. There is also a gyroscopic surveying system whereby the motion of a pendulum is affected by the earth‘s rotation and indicates the direction of geographic north. It supports the measurements made with modern tachymeters. These are capable of measuring a distance of one kilometre with an accuracy of two millimetres. Tachymeters send out infrared waves aimed at reflectors. Surveying the tunnel during construction Surveyor at work with a tachymeter. Measurements are performed at the centre of the tunnel because, due to the temperature of the rock, it is usually warmer at the tunnel wall than it is in the centre of the tunnel. The shape of the earth, which is not an exact sphere due to different density ratios in the earth‘s interior, must also be taken into consideration. As sea water propagates along this geoid, sea level is used as a worldwide reference surface for height measurements. Eliminating possible sources of error Geoid: This representation shows that the Earth is not a perfect sphere. This must be taken into account when taking measurements. A further difficulty is that different official height reference points are used in Austria and Italy. The reference point for the sea in Italy is the sea level at Genoa, for Austria the sea level at Trieste. As this leads to a difference of 12.5 centimetres at Brenner, it was agreed that the construction of the Brenner Base Tunnel would use the Euro- pean height system UELN with Amsterdam as the reference level.