Integrated time-space modelling based on correlated measurements for the determination of survey configurations and the description of deformation processes
Final Report Abstract
Within the project a novel method that allows the statistically based deformation analysis using the epochal and space-continuous estimated B-spline surfaces was presented. Within the project, geometrical changes are described in a model-based way based on measurements of civil structures with terrestrial laser scanners (TLSs). Such an area-wise deformation analysis includes the modelling of the acquired point cloud by means of continuous functions in order to reduce the amount of data by encoding the geometrical information in the function’s parameters. This is especially interesting for structures with complex (curved) shapes. Due to their high flexibility, parametric free form surfaces like B-spline surfaces are suitable to model point clouds of such structures. However, their high flexibility requires the determination of a variety of parameter types such as the number of control points, which directly defines the surface’s complexity, and the allocation of appropriate surface parameters. Much effort has been invested into the development of estimation procedures for this kind of surfaces for TLS point clouds. Parallel to the development of the functional point cloud modelling with B-splines, TLS specific measurement deviations were investigated and stochastically modelled at the project partner of the Institute of Engineering Geodesy Stuttgart (IIGS) of the University of Stuttgart. The resulting stochastic model is a synthetic variance-covariance-matrix (SVCM) that considers three groups of errors according to their correlating nature. It includes uncertainties caused by the measurement instrument (TLS), the surrounding environmental conditions between instrument and measured object, and finally, the object surface properties. The first group includes non-correlating as well as functional correlating errors, whereas the second and third group consists of stochastic correlating errors. The SVCM takes all major TLS error sources into account and describes the existing dependencies (or independencies) between the points and thereafter regions of estimated surfaces. The matrix was then integrated into the estimation of free form surface parameters. Finding appropriate values for variances and covariances in the matrix is still a challenging task. Based on the functional description of point clouds acquired in different epochs, two models that describe geometrical changes of the measuring objects were developed during the second project phase at the Research Group Engineering Geodesy (RGEG) of the Technical University of Vienna. The first model exclusively describes rigid body movements (rotations, translations and scaling). During the project it could be demonstrated that, under certain circumstances, the rigid body movement can be directly retrieved in the free form surfaces’ control points, allowing the application of classical point based methods on continuous and areal descriptions of point clouds. The second model uses a least squares collocation in order to describe the measuring object’s distortions. The non-distorted object is modelled by means of deterministic free form surfaces, whereas the occurring changes are interpreted and modelled as a stochastic signal. In general, identifying rigid body movements and simultaneously superimposed deformations of scanned objects is possible with the introduced strategy. Another advantage resulting from this type of surface-to-surface comparisons is data segregation. Instead of using complete point clouds that can be difficult to handle due to size (e.g. tens or hundreds of millions of points), an equivalent surfaces is used. Finally, optimal scanning parameters for a scanning site as e.g. angular resolution and quality level as well as relative geometry between object and scanning site can be defined base on variance-based sensitivity analysis. This allows to detect deformation, in this case minimal rigid body movements, by an optimal choice of the scanning site. Finally, the practical applicability of all project findings was verified on simulated data sets, real objects measured in laboratory and field conditions. IMKAD gives a solid theoretical basis for areawise deformation analysis. As regards future research, the progress during this project showed that many topics could not be thoroughly investigated due to complexity and time. Open research questions are the time dependence of the stochastic model, extended deformation models with a focus on the separation of stable and unstable areas, a sensitivity analysis that yields an optimized scanning network and, finally investigations with respect to georeferencing models and strategies.
Publications
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(2015): Continuous modelling of point clouds by means of freeform surfaces. Österreichische Zeitschrift für Vermessung und Geoinformation (VGI), Vol. 103, Issue 2+3, pp. 121 - 129
Harmening, C., Neuner, H.
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(2016): Choosing the Optimal Number of B-spline Control Points (Part 1: Methodology and Approximation of Curves). Journal of Applied Geodesy, Vol. 10, Issue 3, pp. 139–157, deGruyter, Berlin
Harmening, C., Neuner, H.
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(2016): First investigations for a synthetic covariance matrix for monitoring by terrestrial laser scanning. In: Proceedings of 3rd Joint International Symposium on Deformation Monitoring (JISDM), 30.03-01.04, 2016, Vienna, Austria
Kauker, S.; Schwieger, V.
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(2016): Spatio-Temporal Correlations of Terrestrial Laser Scanning. Allgem. Verm. Nachr, Vol. 6/2016, S. 170-182, Wichmann Verlag, Berlin
Kauker, S.; Holst, C.; Schwieger, V.; Kuhlmann, H.; Schön, S.
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(2016): Terrestrial Laserscanning - Modeling of Correlations and Surface Deformations. In: Proceedings of FIG Working Week 2016, Christchurch, New Zealand, 2016
Harmening, C.; Kauker, S.; Neuner, H.; Schwieger, V.
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(2017): A synthetic covariance matrix for monitoring by terrestrial laser scanning. Journal of Applied Geodesy, Vol. 11, Issue 2, pp. 77-88, deGruyter, Berlin
Kauker, S.; Schwieger, V.
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(2017): Choosing the Optimal Number of B-spline Control Points (Part 2: Approximation of surfaces and applications). Journal of Applied Geodesy. Vol. 11, Issue 1, pp. 43–52, deGruyter, Berlin
Harmening, C.; Neuner, H.
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(2017): Evaluating the freeform modelling of point clouds by means of a test specimen. In: Kopáčik, A.; Kyrinovič, P.; Henriques, M. (Eds.). In: Proceedings of 7th International Conference on Engineering Surveying, pp. 255-264
Harmening, C.; Teodori, G.; Neuner, H.
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(2017): Modellierung und Auswirkung von Korrelationen bei der Schätzung von Deformationsparametern beim terrestrischen Laserscanning. In: Lienhart, W. (Ed.): In: Proceedings of 18. Internationalen Ingenieurvermessungskurs in Graz 2017, pp. 321-336, Wichmann, Berlin
Kauker, S.; Harmening, C.; Neuner, H.; Schwieger, V.
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(2020): A spatio-temporal deformation model for laser scanning point clouds. Journal of Geodesy, 94 (2020), 26; 1 - 26
Harmening, C.; Neuner, H.
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(2020): Elementary Error Model Applied to Terrestrial Laser Scanning Measurements: Study Case Arch Dam Kops. Mathematics 2020, Vol. 8, Issue 4
Kerekes, G.; Schwieger, V.
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(2021): Determining Variance-Covariance Matrices for Terrestrial Laser Scans: A Case Study of the Arch Dam Kops. In: Kopáčik A., Kyrinovič P., Erdélyi J., Paar R., Marendić A. (eds). Contributions to International Conferences on Engineering Surveying. Springer Proceedings in Earth and Environmental Sciences. Springer, Cham
Kerekes G., Schwieger V.
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(2021): Estimating Control Points for B-Spline Surfaces Using Fully Populated Synthetic Variance–Covariance Matrices for TLS Point Clouds. Remote Sensing. 2021; Vol. 13, Issue 16
Raschhofer, J.; Kerekes, G.; Harmening, C.; Neuner, H.; Schwieger, V.
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(2021): Laser Scanner-Based Deformation Analysis Using Approximating B-Spline Surfaces; Remote Sensing, Vol. 13 (2021), Issue 18
Harmening, C.; Hobmaier, C.; Neuner, H.
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(2022): Two-epoch TLS deformation analysis of a double curved wooden structure using approximating B-spline surfaces and fully-populated synthetic covariance matrices. In: Proceedings of 5th Joint International Symposium on Deformation Monitoring (JISDM), 20.06-22.06.2022, Valencia, Spain
Kerekes G.; Raschhofer, J.; Harmening, C.; Neuner, H.; Schwieger V.
