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Gyro theodolites – briefly gyros – allow the orientation transfer from a reference line to another line without need for a connecting geodetic network. They are routinely used for orientation control of networks in tunnelling projects. This is currently the only way to stabilize accuracy and reliability in tunnelling networks and is indispensable with respect of today’s requirements for excavation and breakthrough accuracy. In order to archive a reliable assessment of correctness, the measuring method has to be planned in a way that systematic influences in the determination of an underground directional angle are minimized. For this purpose, the principle of differential measurements is used for an azimuth determination both in terms of time as well as in space. All required corrections and reductions must be considered correctly to obtain the directional angle of interest with a measurement uncertainty less than 1 mgon (=∧3.3″$\stackrel{\wedge }{=}3.{3^{\prime\prime }}$). Some accuracy considerations obtained from own experiences are discussed to state whether the specified standard deviations of the used gyros according to DIN 18723-7 can be confirmed.
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