Influence of surface reflectivity on reflectorless electronic distance measurement and terrestrial laser scanning

The uncertainty of electronic distance measurement to surfaces rather than to dedicated precisionre flectors (reflectorless EDM) is afected by the entire system comprising instrument, atmosphere and surface. The impact of the latter is significant for applications like geodetic monitoring, high-precision surface modelling or laser scanner self-calibration. Nevertheless, it has not yet received sufficient attention and is not well understood. We have carried out an experimental investigation of the impact of surface reflectivity on the distance measurements of a terrestrial laser scanner. The investigation helps to clarify (i)whether variations of reflectivity cause systematic deviations of reflectorless EDM, and (ii) if so, whether it is possible and worth modelling these deviations. The results show that differences in reflectivity may actually cause systematic deviations of a few mm with diffusely re- flecting surfaces and even more with directionally reflecting ones. Using abivariate 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 reflectivity (or equivalently received signal strength) on the distance measurement and that it is possible to model and predict this effect. Integration into laser scanner calibration models may be beneficial for high precision applications. The results may apply to a broad range of instruments, not only to the specific laser scanner used herei

The SLAC Vertical Comparator for the Calibration of Digital Levels

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 {micro}m, 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

Numerical simulation of a ground anchor pullout test monitored with fiber optic sensors

This paper presents a comprehensive study of the load transfer mechanism of a soil anchor during pullout test. The anchor was monitored with fiber optic sensors installed along the tendon and embedded in the grout, which enabled continuous measurements of strains with a spatial resolution of about 10 mm. The measured strain profile along the tendon indicates that the load is not transferred from the tendon to the grout at the same rate along the fixed length. Crack development was verified by measured peaks along the longitudinal strain profile. The in situ measurements were compared with results from finite-element simulations contributing to the interpretation of the in situ results. The load displacement curves and strain distributions obtained with the numerical simulations compare well with measurements. The constitutive model selected for the grout is capable of reproducing the crack development and the numerical simulations suggest that debonding occurred during the anchor pullout test

Influence of surface reflectivity on reflectorless electronic distance measurement and terrestrial laser scanning

ISSN:1862-9016ISSN:1862-902