Terrestrisches Laserscanning für die Geodätische Deformationsmessung

The determination of geometric changes within an area or object of interest by means of repetitive surveys at different points in time is referred to as geodetic deformation monitoring. Despite its development in the early years of the twentieth century the original processing chain remained identical in essence until now. It contains the choice of suitable viewpoints, observation of at least two so called epochs, transformation of all epochs into a common coordinate system and finally the actual monitoring of deformations. In order to acquire an area under investigation discrete points of interest have to be physically signalised. Thereby repetitive observations can be achieved throughout the epochs. Downsides of this approach are among others the time consuming necessity to signalise the area under investigation as well as the “blindness” against deformations that occur outside the scope of the interest points. The emergence of terrestrial laser scanners (TLS) into engineering geodesy around the turn of the millennium led to a paradigm shift that allows to observe an area under investigation in a quasi-laminar fashion without the need of signalising points within the object space. Through this, all deformations can be revealed in principle that occurred in between two epochs within the area under investigation. Based on the already mentioned process chain of geodetic deformation monitoring the contribution at hand initially compares methodical differences as well as parallels among established approaches and terrestrial laser scanning. This results in several unsolved problems that are treated as research questions in this thesis. A substantial preparative step for geodetic deformation monitoring is the choice of suitable viewpoints from an economic perception and under the perspective of engineering geodesy. As existing methods for this task are not directly transferable to TLS, a novel combinatorial search algorithm is proposed and exemplified. Furthermore, a stochastic model for terrestrial laser scanners is introduced that uses intensity values of the received laser signal as an input and allows to predict the theoretical precision of observations from a certain viewpoint. A vital task in deformation monitoring under the assumption of a congruency model is the transformation into a stable reference frame. Only if this prerequisite holds, occurred deformations can be correctly identified and consequently quantified. For the case of observations onto signalised, discretely observable targets, for instance by tacheometry, several methods have been developed in the past in order to reveal sets of congruent points respectively points that were subject of deformation, so that this problem domain can be seen as solved. If one now transforms this problem to TLS then it can be established that areas where deformation occurred have to be identified and rejected from computing transformation parameters between epochs. A look at current literature on TLS based deformation monitoring shows that nearly all researchers imitate the tacheometric line of action by deploying artificial targets which have been designed for laser scanners. Through this a beneficial characteristic of TLS is neglected namely the enormous information density within the object space that can be used to compute transformation parameters. For the until then unsolved problem of automatically distinguishing stable and deformed regions in datasets which have been captured by TLS two algorithms are proposed. The performance of these implementations is tested regarding their robustness and other criteria, based on practical data. Furthermore, a method for determination of statistically significant deformations in TLS-datasets is introduced. Through this, the subjective choice of arbitrary thresholds for quantification and visualisation of deformations is counteracted. Finally, a procedure for visualisation of deformations within the object space is presented that simplifies the now and then abstract interpretation of the outcome of deformation monitoring.Die Bestimmung von geometrischen Veränderungen eines Untersuchungsgebietes bzw. -objektes durch wiederholte Vermessung zu verschiedenen Zeitpunkten wird als geodätische Deformationsmessung bezeichnet. Auch seit deren methodischen Entwicklung zu Beginn des zwanzigsten Jahrhunderts blieb die ursprüngliche Prozesskette im Wesentlichen unverändert und beinhaltet die Wahl von geeigneten Aufnahmestandpunkten, die Vermessung von mindestens zwei sogenannten Epochen, die Überführung der Epochen in ein gemeinsames Koordinatensystem und schließlich die eigentliche Deformationsmessung. Zur Erfassung einer Epoche wird das Untersuchungsgebiet zunächst diskretisiert, indem interessierende Punkte signalisiert werden, um so eine wiederholte Vermessung zu ermöglichen. Nachteile dieser Vorgehensweise sind unter anderem die zeitaufwändige Signalisierung, sowie die „Blindheit“ gegenüber Deformationen, die in nicht signalisierten Arealen aufgetreten sind. Mit Einzug des terrestrischen Laserscannings (TLS) in die Ingenieurgeodäsie wurde um die Jahrtausendwende ein Paradigmenwechsel eingeleitet, der nun eine quasi-flächenhafte Vermessung ohne die Einbringung von Zielmarken in den Objektraum ermöglicht. Dadurch können prinzipiell alle Deformationen aufgedeckt werden, die zwischen zwei Epochen im Untersuchungsgebiet aufgetreten sind. Die vorliegende Arbeit vergleicht zunächst an Hand der bereits erwähnten Prozesskette der geodätischen Deformationsmessung methodische Unterschiede sowie Parallelen zwischen der etablierten Vorgehensweise und dem terrestrischen Laserscanning. Auf Grundlage der ermittelten ungelösten Probleme ergeben sich die Forschungsfragen der weiteren Kapitel. Ein wesentlicher Schritt zur Vorbereitung von geodätischen Deformationsmessungen ist die Auswahl geeigneter Aufnahmestandpunkte unter ökonomischen und ingenieurgeodätischen Gesichtspunkten. Da bestehende Verfahren nicht direkt auf das terrestrische Laserscanning angewendet werden können, wird ein neuartiger modellbasierter Algorithmus zur kombinatorischen Suche geeigneter Aufnahmestandpunkte vorgestellt und an einem Beispiel demonstriert. Zudem wird ein stochastisches Modell für terrestrische Laserscanner vorgestellt, welches als Eingangsgrößen Intensitätswerte des reflektierten Lasersignals verwendet und somit die Berechnung der zu erwartenden Präzision der Messungen von einem vorgegebenen Standpunkt ermöglicht. Ein entscheidender Punkt bei der Deformationsmessung unter Annahme eines Kongruenzmodells bildet die Überführung einzelner Epochen in ein stabiles Referenzkoordinatensystem. Nur wenn diese Annahme erfüllt wird, gelingt es, aufgetretene Deformationen korrekt zu identifizieren und schließlich zu quantifizieren. Liegen Beobachtungen zu signalisierten, diskret anzielbaren Zielzeichen vor, stehen zahlreiche Methoden zur Verfügung, um kongruente Punktgruppen zu ermitteln, so dass dieses Problem als gelöst angesehen werden kann. Überträgt man dieses Problem auf das terrestrische Laserscanning, so gilt es nun, flächenhafte Areale zu erkennen in denen Deformationen aufgetreten sind, und diese von der Berechnung von Transformationsparametern zwischen den Epochen auszuschließen. Ein Blick in aktuelle Publikationen zum Thema Deformationsmessung mit TLS zeigt, dass nahezu alle Ansätze die tachymetrische Herangehensweise durch Nutzung von Zielzeichen imitieren, die in den Objektraum eingebracht werden müssen. Dadurch wird ein wesentlicher Vorteil von terrestrischen Laserscannern vernachlässigt, nämlich die enorm hohe Informationsdichte im Objektraum, die zur Berechnung von Transformationsparametern genutzt werden kann. Für das bis dahin ungelöste Problem der automatischen Identifikation von stabilen und deformierten Regionen in Datensätzen aus TLS werden zwei Algorithmen vorgestellt. Die Leistungsfähigkeit der Algorithmen wird hinsichtlich der Robustheit gegenüber Deformationen und weiterer Kriterien an verschiedenen praktischen Szenarien getestet. Des Weiteren wird eine Methode zur Ermittlung von statistisch signifikanten Deformationen in TLS-Datensätzen vorgestellt, wodurch der subjektiven Wahl von frei wählbaren Schwellwerten bei der Quantifizierung und Visualisierung von Deformationen begegnet wird. Schließlich wird ein Verfahren zur Visualisierung von Deformationen im Objektraum präsentiert, welches die mitunter abstrakte Interpretation der Ergebnisse einer Deformationsmessung erleichtert.BMBF, 17N0509, Implementierung neuer 3D-Matchingverfahren für den Einsatz Terrestrischer Laserscanner (TLS) in der Deformationsmessun

Determination of Intensity-Based Stochastic Models for Terrestrial Laser Scanners Utilising 3D-Point Clouds

Recent advances in stochastic modelling of reflectorless rangefinders revealed an inherent relationship among raw intensity values and the corresponding precision of observed distances. In order to derive the stochastic properties of a terrestrial laser scanner’s (TLS) rangefinder, distances have to be observed repeatedly. For this, the TLS of interest has to be operated in the so-called 1D-mode—a functionality which is offered only by a few manufacturers due to laser safety regulations. The article at hand proposes two methodologies to compute intensity-based stochastic models based on capturing geometric primitives in form of planar shapes utilising 3D-point clouds. At first the procedures are applied to a phase-based Zoller + Fröhlich IMAGER 5006h. The generated results are then evaluated by comparing the outcome to the parameters of a stochastic model which has been derived by means of measurements captured in 1D-mode. Another open research question is if intensity-based stochastic models are applicable for other rangefinder types. Therefore, one of the suggested procedures is applied to a Riegl VZ-400i impulse scanner, as well as a Leica ScanStation P40 TLS that deploys a hybrid rangefinder technology. The generated results successfully demonstrate alternative methods for the computation of intensity-based stochastic models as well as their transferability to other rangefinder technologies

Permanent Terrestrial LiDAR Monitoring in Mining, Natural Hazard Prevention and Infrastructure Protection – Chances, Risks, and Challenges: A Case Study of a Rockfall in Tyrol, Austria

[EN] The objective of this work is the development of an integrated monitoring service for the identification and evaluation of ground surface and slope movements in the context of coal mining, the prevention of natural hazards and protection of infrastructure. The focus is set on the integration of a long-range terrestrial laser scanner into a continuous monitoring system from an engineering geodetic point of view. In the Vals valley in Tyrol, a permanently installed laser scanner was successfully operated via a web portal to monitor surface processes in the area of rockfall debris on a high-mountain slope in the summers of 2020 and 2021. This paper describes the practical benefits of this permanent laser scanning installation. In addition to the potentials of automatic data acquisition, possibilities for multitemporal analysis with respect to spatio-temporally variable changes are presented, using advanced 3D change detection with Kalman filtering. The level of detection for deformation analyses therein depends on the quality of the georeferencing of the sensor and the noise within the measured point cloud. We identify and discuss temporally variable artifacts within the data based on different methods of georeferencing. Finally, we apply our change detection method on these multitemporal data to extract specific information regarding the observed geomorphologic processes.We would like to thank the Tyrol State Government - Department of Geoinformation for their support in conducting the experimental study. Many thanks to the Central Institute for Meteorology and Geodynamics (ZAMG) for providing the weather data. The measurement setup is supported by the European Union Research Fund for Coal and Steel [RFCS project number 800689 (2018)].Schröder, D.; Anders, K.; Winiwarter, L.; Wujanz, D. (2023). Permanent Terrestrial LiDAR Monitoring in Mining, Natural Hazard Prevention and Infrastructure Protection – Chances, Risks, and Challenges: A Case Study of a Rockfall in Tyrol, Austria. Editorial Universitat Politècnica de València. 51-59. https://doi.org/10.4995/JISDM2022.2022.13649515

Determination of Intensity-Based Stochastic Models for Terrestrial Laser Scanners Utilising 3D-Point Clouds

Recent advances in stochastic modelling of reflectorless rangefinders revealed an inherent relationship among raw intensity values and the corresponding precision of observed distances. In order to derive the stochastic properties of a terrestrial laser scanner’s (TLS) rangefinder, distances have to be observed repeatedly. For this, the TLS of interest has to be operated in the so-called 1D-mode—a functionality which is offered only by a few manufacturers due to laser safety regulations. The article at hand proposes two methodologies to compute intensity-based stochastic models based on capturing geometric primitives in form of planar shapes utilising 3D-point clouds. At first the procedures are applied to a phase-based Zoller + Fröhlich IMAGER 5006h. The generated results are then evaluated by comparing the outcome to the parameters of a stochastic model which has been derived by means of measurements captured in 1D-mode. Another open research question is if intensity-based stochastic models are applicable for other rangefinder types. Therefore, one of the suggested procedures is applied to a Riegl VZ-400i impulse scanner, as well as a Leica ScanStation P40 TLS that deploys a hybrid rangefinder technology. The generated results successfully demonstrate alternative methods for the computation of intensity-based stochastic models as well as their transferability to other rangefinder technologies