Efficient calibration strategies for panoramic terrestrial laser scanners

Terrestrial laser scanner (TLS) measurements are unavoidably affected by systematic errors due to internal mechanical misalignments, while the calibration is the fundamental procedure for mitigating these effects and assuring the high measurement accuracy. Despite different existing calibration solutions, the practice of calibrating TLSs for the sake of quality assurance is rarely exercised from the majority of end-users in the commercial sector. The main reason is the inefficiency of the existing solutions requiring considerable investments of time, funding and effort. Therefore, this work is dedicated to increasing the efficiency of the existing calibration approaches, development of further efficient calibration approaches that overcome some shortcomings of the existing ones, and aiding the selection of the appropriate calibration strategy based on the analysis of pros and cons. Generally, the calibration approaches can be divided on the a priori calibration, before the measurement campaign, and the in-situ calibration, during the measurement campaign, each having certain advantages and disadvantages. Hence, this work incorporates the investigation of both strategies, as well as their analysis and comparison, and, therefore, it can be separated into three main aspects:  For pursuing the efficient a priori TLS calibration, this works relies on the well-established target-based self-calibration approach, which suffers from inefficiency, as it requires numerous manually installed targets and multiple scans to assure accurate calibration results. Therefore, herein, based on the detailed analysis of the functional and stochastic model of the calibration adjustment, as well as the sensitivity analysis of different measurement configurations, an optimized calibration field design is derived. The results led to the implementation of the first non-commercial facility for the comprehensive calibration of high-end panoramic TLSs in Europe. It comprises only a handful of targets and requires typically one hour for the calibration procedure, while at the same time assures trustworthy calibration results. However, the main contribution lies in the simple designing workflow, which can be easily reproduced by qualified engineers for establishing new efficient calibration fields.  As the stability of the calibration parameters is found to be one of the limiting factors of the a priori calibration, further development of the in-situ calibration approaches was pursued. Different calibration strategies are analyzed on the concrete case of deformation monitoring of VLBI telescope’s main reflector, pointing out the requirements of the successful calibration: bundled calibration adjustment with sufficient variations in the measurement configuration and the explicit inclusion of two-face measurements. Additionally, the first fully automatic in-situ calibration approach was introduced in the response to the efficiency related shortages of the existing in-situ calibration strategies identified in the latter investigation.  The advantages and disadvantages of the latter a priori and in-situ calibration approaches were analyzed in detail to aid the decision-making process of the end-users and to help their integration in the usual workflows in the commercial sector. Within the analysis, it was proven that the user-calibration of instruments can indeed improve the point cloud accuracy beyond the manufacturer’s initial factory calibration. Moreover, it is proven that, if necessary, user-calibration can even substitute the manufacturer’s calibration entirely. However, the highest measurement accuracy is assured when the manufacturer’s calibration, a priori calibration and in-situ calibration are adequately combined. In the overall view, the findings and methods developed in this thesis contribute to the integration of the TLS calibration practices in the commercial applications and aids their wider acceptance. Therefore, this work represents a relevant contribution to the superordinate goal of quality assurance of TLS measurements.