Keypoint-based deformation monitoring using a terrestrial laser scanner from a single station: Case study of a bridge pier

[EN] Terrestrial laser scanners (TLSs) offer a possibility for more automated and efficient deformation monitoring of civil engineering structures with higher spatial resolution than standard methods, as well as without the necessity of permanently installing the monitoring equipment. In such applications, scanners are usually placed so that the lines of sight are roughly aligned with the main directions of the expected deformations, and the deformations are estimated from point cloud differences between multiple epochs. This allows high sensitivity in the direction of the surface normal, but deformations along the surface are often undetected or hard to precisely quantify. In this work, we propose an algorithm based on the detection and matching of keypoints identified within TLS intensity images. This enables precise quantification of deformations along the scanned surfaces. We also present the application of the algorithm for monitoring a bridge pier of the Hochmoselbrücke in Germany, as a case study. Deformations up to about 4 cm due to thermal expansion and bending of the pier were successfully detected from scans taken throughout the day from a single location, up to 180 m from the monitored surfaces. The results agreed within a few millimeters to independent monitoring using state-of-the-art processing of TLS point clouds obtained from a different location and using a different type/brand of instrument. The newly proposed algorithm can either be used to complement existing TLS-based deformation analysis methods by adding sensitivity in certain directions, or it can be valuable as a standalone solution.Medic, T.; Ruttner, P.; Holst, C.; Wieser, A. (2023). Keypoint-based deformation monitoring using a terrestrial laser scanner from a single station: Case study of a bridge pier. En 5th Joint International Symposium on Deformation Monitoring (JISDM 2022). Editorial Universitat Politècnica de València. 167-175. https://doi.org/10.4995/JISDM2022.2022.1381216717

Keypoint-based deformation monitoring using a terrestrial laser scanner from a single station: Case study of a bridge pier

errestrial laser scanners (TLSs) offer a possibility for more automated and efficient deformation monitoring of civil engineering structures with higher spatial resolution than standard methods, as well as without the necessity of permanently installing the monitoring equipment. In such applications, scanners are usually placed so that the lines of sight are roughly aligned with the main directions of the expected deformations, and the deformations are estimated from point cloud differences between multiple epochs. This allows high sensitivity in the direction of the surface normal, but deformations along the surface are often undetected or hard to precisely quantify. In this work, we propose an algorithm based on the detection and matching of keypoints identified within TLS intensity images. This enables precise quantification of deformations along the scanned surfaces. We also present the application of the algorithm for monitoring a bridge pier of the Hochmoselbrücke in Germany, as a case study. Deformations up to about 4 cm due to thermal expansion and bending of the pier were successfully detected from scans taken throughout the day from a single location, up to 180 m from the monitored surfaces. The results agreed within a few millimeters to independent monitoring using state-of-the-art processing of TLS point clouds obtained from a different location and using a different type/brand of instrument. The newly proposed algorithm can either be used to complement existing TLS-based deformation analysis methods by adding sensitivity in certain directions, or it can be valuable as a standalone solution

Vibration monitoring of a bridge using 2D profile laser scanning: Lessons learned from the comparison of two spatio-temporal processing strategies

Profile laser scanning allows sub-millimeter precise contact-free measurements with high spatial and temporal resolution. That makes it an appealing solution for structural health monitoring focusing on vibrations of engineering structures, such as the analysis of eigenmodes and eigenfrequencies of bridges. In this work, we use the profile scanning mode of a Zoller+Fröhlich Imager 5016 terrestrial laser scanner (TLS) to observe bridge dynamics, focusing on the free decay processes following trains passing the bridge and exciting the structure. We compare two vibration monitoring strategies and implement an open-source semi-automatic software that integrates both approaches. We successfully estimate a spatio-temporal vibration model (including dampening coefficient) despite the maximum vibration amplitude reaching only 0.3 mm during the free decay process. Both strategies allow the estimation of the first eigenfrequency with a precision better than 0.1 Hz. Within the paper, we highlight the advantages and tackle the identified challenges of these vibration monitoring strategies. We also report on a preliminary investigation of appropriate instrument positioning for estimating the parameters of a spatio-temporal vibration model

Decreasing the Uncertainty of the Target Center Estimation at Terrestrial Laser Scanning by Choosing the Best Algorithm and by Improving the Target Design

During the registration and georeferencing of terrestrial laser scans, it is common to use targets to mark discrete points. To improve the accuracy of the registration, the uncertainties of the target center estimation (TCE) have to be minimized. The present study examines different factors influencing the precision of the TCE. Here, the focus is on the algorithm and the target design. It is determined that, in general, the uncertainties of the TCE are much smaller than those indicated by the manufacturers. By comparing different algorithms for the first time, it was possible to clearly determine that an algorithm using image correlations yields the smallest standard deviations for the TCE. A comparison of different target designs could not identify an ideal commercially available target. For this reason, a new target, the BOTA8 (BOnn TArget with 8-fold pattern) was developed, which leads to smaller standard deviations than the previous targets. By choosing the best algorithm and improving the target design, standard deviations of 0.5 mm in distance direction and 1.2 arcsec in angular direction for a scan distance up to 100 m were achieved with the laser scanner Leica ScanStation P20. The uncertainties could be reduced by several millimetres and angular seconds compared to the manufacturer’s targets and software

Vibration monitoring of a bridge using 2D profile laser scanning: Lessons learned from the comparison of two spatio-temporal processing strategies

[EN] Profile laser scanning allows sub-millimeter precise contact-free measurements with high spatial and temporal resolution. That makes it an appealing solution for structural health monitoring focusing on vibrations of engineering structures, such as the analysis of eigenmodes and eigenfrequencies of bridges. In this work, we use the profile scanning mode of a Zoller+Fröhlich Imager 5016 terrestrial laser scanner (TLS) to observe bridge dynamics, focusing on the free decay processes following trains passing the bridge and exciting the structure. We compare two vibration monitoring strategies and implement an open-source semi-automatic software that integrates both approaches. We successfully estimate a spatio-temporal vibration model (including dampening coefficient) despite the maximum vibration amplitude reaching only 0.3 mm during the free decay process. Both strategies allow the estimation of the first eigenfrequency with a precision better than 0.1 Hz. Within the paper, we highlight the advantages and tackle the identified challenges of these vibration monitoring strategies. We also report on a preliminary investigation of appropriate instrument positioning for estimating the parameters of a spatio-temporal vibration model.Meyer, N.; Schmid, L.; Wieser, A.; Medic, T. (2023). Vibration monitoring of a bridge using 2D profile laser scanning: Lessons learned from the comparison of two spatio-temporal processing strategies. Editorial Universitat Politècnica de València. 177-184. https://doi.org/10.4995/JISDM2022.2022.1381317718

Supercontinuum-based hyperspectral laser scanning: towards enhanced 3D surface reconstruction and its benefits for remote sensing

We demonstrate a supercontinuum-based hyperspectral laser scanning technique that provides high-precision distance measurements of natural surfaces along with their reflectance signature over the broad spectral range of the supercontinuum (SC) output. The SC used in our experiments is spectrally broadened to 570-970 nm from a 780 nm mode-locked femtosecond laser. Distance measurements are carried out by monitoring the differential phase delay of the intermode beat notes obtained from direct photodetection of the SC, while the backscattered reflection spectrum is acquired using a commercial spectrometer. We achieve a single-point range precision below 10 μm on natural targets (gypsum board and leaves of a plant used herein) placed at a stand-off distance of 5 m. Our results demonstrate the acquisition of hyperspectral point clouds together with sub-mm range noise on the scanned surface. This range performance is comparable to commercial state-of-the-art terrestrial laser scanners which traditionally employ a monochromatic laser source.We show the benefit of enhanced range precision toward correctly estimating the surface orientation and for radiometric calibration of the acquired intensities. Initial results illustrate the direct 3D mapping of spectral data of plant leaves with a reduced angle of incidence-related bias, highlighting new opportunities for future research into remote sensing of vegetation

Finding the Best TLS Point Cloud Registration Algorithm for Long-Range Geomonitoring

Accurate registration of TLS (Terrestrial Laser Scanner) point clouds is essential for unbiased determination of deformations in long-range geomonitoring. We evaluated the performance of several established registration methods in an ongoing geomonitoring case study. Our results showed that the ICP (Iterative Closest Point)-based methods specifically developed with geomonitoring in mind perform the best. However, when dealing with distortions in point clouds, the newly developed stripe-wise non-rigid transformation based on the F2S3 (feature to feature supervoxel-based spatial smoothing) algorithm outperformed all established methods

Alpine Metrology Lab: Geomonitoring Using Long- Range TLS and Permanent GNSS

Due to increasing importance of monitoring geologically active regions, such as landslides, we established an Alpine Metrology Lab (AML) for investigating and further improving geomonitoring technologies. So far, we focus on using permanent GNSS and terrestrial laser scanning (TLS) for validating and enhanced interpretation of InSAR observations. Within this paper we present our initial results related to cross-validation of mass movements observed by permanent GNSS and long-range TLS. The AML is established in the Matter Valley (Switzerland) in a region contaning active landslides and rockglaciers. Our results indicate that long-range TLS is suitable for remote sensing of mass movements over wide areas if the displacements exceed 0.4 m; we detected this as the sensitivity limit of the implemented workflow. The installed permanent GNSS network provided the reference values of mass movement magnitudes with an uncertainty of about 2 to 4 mm, uncovering small systematic biases in the TLS data. This needs to be further investigated in order to use both measurement techniques for validation of InSAR measurements

Increasing Spatio-Temporal Resolution for Monitoring Alpine Solifluction Using Terrestrial Laser Scanners and 3D Vector Fields

This article investigates the usage of terrestrial laser scanner (TLS) point clouds for monitoring the gradual movements of soil masses due to freeze–thaw activity and water saturation, commonly referred to as solifluction. Solifluction is a geomorphic process which is characteristic for hillslopes in (high-)mountain areas, primarily alpine periglacial areas and the arctic. The movement can reach millimetre-to-centimetre per year velocities, remaining well below the typical displacement mangitudes of other frequently monitored natural objects, such as landslides and glaciers. Hence, a better understanding of solifluction processes requires increased spatial and temporal resolution with relatively high measurement accuracy. To that end, we developed a workflow for TLS point cloud processing, providing a 3D vector field that can capture soil mass displacement due to solifluction with high fidelity. This is based on the common image-processing techniques of feature detection and tracking. The developed workflow is tested on a study area placed in Hohe Tauern range of the Austrian Alps with a prominent assemblage of solifluction lobes. The derived displacements were compared with the established geomonitoring approach with total station and signalized markers and point cloud deformation monitoring approaches. The comparison indicated that the achieved results were in the same accuracy range as the established methods, with an advantage of notably higher spatial resolution. This improvement allowed for new insights considering the solifluction processes