Special Issue: Deformation Monitoring

Deformation monitoring is a crucial contribution of geodesy to modern society and the economy. It provides objective and reliable information for a better understanding of critical processes in the natural and man-made environment so that risk assessment and timely adoption of appropriate measures are based on rigorous and scientifically sound methodologies.

This special issue on deformation monitoring contains nine peer-reviewed papers selected by the Scientific Committee of the 5th Joint International Symposium on Deformation Monitoring (JISDM), held in Valencia, Spain, from 20th to 22nd June 2022. The JISDM series, which is the most relevant worldwide event to disseminate new advances in geodetic monitoring and deformation analysis, started in Hong Kong in 2011 and was successfully followed by Nottingham, in 2013, Vienna, in 2016, Athens, in 2019, and more recently, Valencia, in 2022. This last edition was jointly organized by the Higher Technical School of Geodetic, Cartographic and Surveying Engineering and the Department of Cartographic Engineering, Geodesy and Photogrammetry of the Universitat Politècnica de València (UPV). The JISDM 2022 included 90 oral presentations and 18 posters organized in 15 technical sessions with topics related to technical, methodological, and practical advances in the field of deformation monitoring tackling landslides, debris flows and rockfalls, large engineering structures, and last but not least, tangible cultural heritage in archaeology and architecture.

The nine papers contained in this issue, which have passed an independent peer-review process organized by the journal, present three advances concerning single-point definition techniques and six applications based on varied area-wise techniques such as Synthetic Aperture Radar (SAR), Photogrammetry or Terrestrial Laser Scanning (TLS).

In the first paper entitled ‘High-precision intermode beating electro-optic distance measurement for mitigation of atmospheric delays’ by Pabitro Ray, David Salido-Monzú and Andreas Wieser from ETH Zurich, Switzerland, a new instrumental basis for high-accuracy multispectral EDM using an optical supercontinuum to enable dispersion-based inline refractivity compensation is presented. The results reported indicate that absolute distances, i.e. SI-metre traceable, can be obtained with a precision better than 0.05 mm at 50 m, and also that the approach can be further developed for longer distances under outdoor conditions.

The paper ‘EDM-GNSS distance comparison at the EURO5000 calibration baseline: preliminary results’, describes a joint effort by Kinga Wezka, Dominik Próchniewicz, Ryszard Szpunar, and Janusz Walo from the Faculty of Geodesy and Cartography, Warsow, Poland and by Luis García-Asenjo, Sergio Baselga, Pascual Garrigues, and Raquel Luján from the Department of Cartographic Engineering, Geodesy and Photogrammetry, Universitat Politècnica de València, Spain, to prepare the novel primary reference baseline EURO5000 as part of the European Research project GeoMetre to both validate refractivity-compensated EDM prototypes and investigate the metrological traceability of GNSS-based distances with an aimed uncertainty of 1 mm at 5 km (k = 2). The result of this metrological experiment may significantly contribute to overcome the limitations of current high-precision deformation monitoring applications that require their scale to be consistent with the SI-metre within 0.1 ppm in several km.

The third paper, entitled ‘A mobile robot for monitoring floor flatness in real-time’ by Christoph Naab from Karlsruhe Institute of Technology (KIT), Germany, describes the development of a novel robot for height measurement in combination with a stationary tacheometer, for structural inspection of flatness according to DIN 18202:2019-07. The entire process of flatness control is carried out automatically and the results show that height measurements in the lowest millimeter range are fulfilled with a significant gain in performance compared to the traditional manual and time-consuming process based on levelling instruments and tacheometers.

In the fourth paper ‘On the quality checking of persistent scatterer interferometry by spatial-temporal modelling’ by Mohammad Omidalizarandi, Bahareh Mohammadivojdan, Hamza Alkhatib, Jens-André Paffenholz, and Ingo Neumann from Geodetic Institute, Leibniz University Hannover, Germany, the Persistent Scatterer Interferometry (PSI) technique is applied using open-source Synthetic Aperture Radar (SAR) data from the satellite Sentinel-1. In particular, they present a two-step process to overcome the current lack of a rigorous quality assurance process to judge the significance of deformations obtained by using this area-wise technique. A spatio-temporal quality model of the PS data for areas of interest in the city of Hamburg is performed and the results of the deformation analysis are then compared with those provided by the Federal Institute for Geosciences and Natural Resources (BGR), Germany.

Fernando Soares and Vinicius Barbon, from the Faculty of Sciences of the University of Lisbon, Portugal, present the paper ‘Image segmentation of breakwater blocks by edge-base Hough transformation’. In this work, the authors investigate the problem of detecting block displacements in a rubble mound breakwater, which is critical, for they make harbors more vulnerable to waving hazard. Based on 3D point clouds collected by using synchronized time-of-flight sensors and photogrammetry, this study proposes the use of an edge-based image segmentation method, based on the Hough transformation algorithm. The results demonstrate that the proposed image segmentation methodology improves the monitoring of block displacements in terms of accuracy and also reduces the required analysis time.

Since TLS has become an important technique for deformation monitoring, the following four papers discuss important aspects which need to be considered if TLS observations are employed for deformation monitoring.

The first TLS paper by Berit Jost, Daniel Coopmann, and Heiner Kuhlmann from the Institute of Geodesy and Geoinformation, University of Bonn, Germany and by Christoph Holst from the Chair of Engineering Geodesy, Technical University of Munich, Germany, focus on the insufficient knowledge about the error budget and correlations when TLS measurements are required for deformation analyses with high accuracy demands. Their work, entitled ‘Real movement or systematic errors? – TLS-based deformation analysis of a concrete wall’ shows that correlations caused by systematic errors in the scanner exist and they can lead to a misinterpretation as a deformation of the object. As a case study they analyze the deformation of a concrete wall.

In the second TLS paper, entitled ‘Investigation of space-continuous deformation from point clouds of structured surfaces’, Elisabeth Ötsch and Hans Neuner from the Department of Geodesy and Geoinformation, TU Vienna, Austria along with Corinna Harmening from Karlsruhe Institute of Technology, Germany, investigate how discontinuities of the object’s surface resulting from artificial structures, e.g. single bricks or concrete blocks, influence the parameters of the approximated continuous surfaces and thus, the derived deformation. Their approach, based on surfaces approximation by using B-splines under a Gauss-Markov-Model, allows the proper use of the measurement stochastic properties in the deformation analysis. Among other advantages, this approach ensures the applicability of the well-established congruency model.

The third TLS paper, entitled ‘Supervoxel-based targetless registration and identification of stable areas for deformed point clouds’ by Yihui Yang and Volker Schwieger from the Institute of Engineering Geodesy, University of Stuttgart, Germany, tackles the TLS problem that arises in scenarios with partially unstable areas where automatic and robust point cloud registration can be significantly affected by the lack of signalized targets. They proposed a fully automatic registration algorithm for point clouds which does not require artificial targets or extracted feature points. In comparison with existing methods, i.e. voxel-based method and the variants of the iterative closest point (ICP) algorithm, the results obtained by using the proposed algorithm show a clear improvement of the registration accuracy and therefore, the deformation can be detected with a higer level of significance.

The authors of the last contribution are Michael J. Olsen, Chris Massey, Ben Leshchinsky, Joseph Wartman, and Andrew Senogles from the Oregon State University, USA, with the paper entitled ‘Forecasting post-earthquake rockfall activity’ describing how geomorphic processes as well as rockfall activity related to seismic activity can be documented and quantified, respectively, by using a database of repeat TLS collected during earthquake sequences. In particular, they present a case study in Canterbury, New Zealand, which focuses on rock slopes, one of them located adjacent to a main highway. The authors conclude that forecasting models developed from the correlation between the activity rate and failure depths with seismic intensity measures can contribute in assisting maintenance needs for debris removal so as to minimize road closures from rockfalls after seismic events.

I would like to thank Prof. Chris Rizos and Prof. Hans-Berndt Neuner for offering the opportunity to publish these nine conference papers in the JAG. In particular, I want to express my profound gratitude to Prof. Neuner for his support and invaluable help during the whole process of the JISDM organization. Considering the high quality of the JISDM 2022 presentations, I encourage the research groups to publish their work in this journal and to attend future JISDM to disseminate the field of deformation monitoring.

Luis García-Asenjo

Guest Editor