Digital levels replaced spirit levels in most fields of precise height measurements because of the automation of the height readings. Three manufacturers offer digital levels with a single reading resolution of 10 µm, and for all of them systematic effects are known. In Europe several facilities for system calibration of digital levels using vertical comparators were established within the last decade. However, there still was no system calibration facility in North America. In order to guarantee the accuracy required for the alignment of experiments at the Stanford Linear Accelerator Center (SLAC) a calibration facility for the system calibration of digital levels was built. In this paper the setup of the SLAC vertical comparator is described in detail and its standard uncertainty is derived. In order to perform traditional rod calibration of conventional line-scaled rods, a CCD camera was integrated into the SLAC comparator. The CCD camera setup is also briefly described. To demonstrate the capabilities of the comparator, results of system and rod calibration are shown.

Abstract:The uncertainty of electronic distance measurement to surfaces rather than to dedicated precisionre ectors (re ectorless EDM) is a ected by the entire system comprising instrument, atmosphere and surface. The impact of the latter is signi cant for applications like geodetic monitoring, high-precision surface modelling or laser scanner self-calibration. Nevertheless, it has not yet received su cient attention and is not well understood. We have carried out an experimental investigation of the impact of surface re ectivity on the distance measurements of a terrestrial laser scanner. The investigation helps to clarify (i) whether variations of re ectivity cause systematic deviations of re ectorless EDM, and (ii) if so, whether it is possible and worth modelling these deviations. The results show that di erences in re ectivity may actually cause systematic deviations of a few mm with di usely reecting surfaces and even more with directionally re ecting ones. Using a bivariate quadratic polynomial we were able to approximate these deviations as a function of measured distance and measured signal strength alone. Using this approximation to predict corrections, the deviations of the measurements could be reduced by about 70% in our experiment. We conclude that there is a systematic effect of surface re ectivity (or equivalently received signal strength) on the distance measurement and that it is possible to model and predict this e ect. Integration into laser scanner calibration models may be bene cial for highprecision applications. The results may apply to a broad range of instruments, not only to the speci c laser scanner used herein.

Reinforced earth structures are commonly used to construct earth structures with high inclination angles. The stability and safety of such constructions is highly dependent on the strain distribution of the geogrids inside the earth structure. In this article, we report about the development and implementation of a structural monitoring system to measure the strain distribution of geogrids with high accuracy and high spatial resolution. The system was developed for the Semmering Base Tunnel project in Austria but can also be applied to other reinforced earth structures. The Semmering Base Tunnel is one of the core infrastructure projects in central Europe. Prior to the start of the actual tunnel excavation, comprehensive preparatory works in the alpine landscape were necessary. One of these works included the relocation of a river stream at the disposal site Longsgraben. To establish this relocation, a reinforced earth structure with a total length of more than 1.3 km and heights up to 25 m was constructed. To evaluate the stability of this structure, a monitoring system was developed. This monitoring system is thereby based on external geodetic and internal distributed fiber optic measurements. For the latter, about 2 km of Brillouin sensing cables in several sensor sections were installed in the project area. In this paper, we report about the laboratory experiments to determine strain and temperature coefficients for the conversion of the measured Brillouin frequency shifts into strain or temperature values. Furthermore, the long-term behavior of the monitoring structure is analyzed for the period of 1 year after installation.

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