Rhätische Bahn, a privately owned rail-transport company in Switzerland, instructed the surveyors’ office of Grünenfelder & Partners in Domat-Ems (Switzerland) to carry out a series of geodetic surveys over an eight-year period at the south portal of the Vereina tunnel. Its purpose is to monitor deformation and settlement of major elements in the large and steadily growing station: retaining walls, tunnel walls, inclinometer sites, etc.
The contract required a network of reference points that would adapt as efficiently and economically as possible, but without loss of accuracy or reliability, to a constantly increasing number of surveyed control points. It also called for new reference and backup points to be established ahead of time to replace those no longer available as construction work progressed. The specified positional and height accuracy was 0.5mm to 1.5mm. Changeable meteorological conditions and in some cases poor visibility complicate field work.
We had previously done three surveys a year, using a Leica NA2000 digital level and a TC2002 total station, two precision prisms, and special targets designed by Grünenfelder & Partners.
Now, for the first time on this project, we used the new Leica TCA1800 with ATR automatic target recognition. The results were impressive.
The method in brief: We used the ATR program developed by Leica for serial measurements. In this, for the first half set – the so-called learning stage – the surveyor points the instrument approximately by hand at each target and starts the automatic fine-pointing procedure. The motor-driven instrument then makes all the rest of the measurements automatically. When it measures a mix of targets, i.e. those with and those without reflectors, the TCA1800 stops the automatic program when it points at a target without a reflector, waits for the surveyor to make the measurement manually, and then continues automatic measurement to the other targets.
By using automatic target recognition for serial measurements with the TCA1800 in the practical example of the Vereina tunnel, we found we could reduce the time taken to less than a third compared with conventional procedures. In these, serial measurements and distance measurements are usually separate processes, but ATR measures both distance and angles. This not only saves time; the additional measurements also increase the reliability and accuracy of network adjustment. In this particular case, we were able to adjust the network in position and height free of constraints.
We achieved the following accuracies:
- Distance s 1.6mm per km.
- Horizontal bearings 0.0004gon.
- Height difference 9.0mm per km.
Because automatic target recognition as such is little used in deformation surveys, the main emphasis in this case was on automatic fine pointing. Our most important experience in the practical example of the Vereina tunnel was that with proper attention to basic aspects – centring, determination of instrument height, adjustment of the operational instrument to the ambient temperature, and reduction of refraction problems to a minimum – the results that automatic fine pointing produces are as accurate as those obtainable in conventional precision surveys, but with a significant saving of time
