Tachymetric Measurements Research Articles

Designing a state‐of‐the‐art monitoring system in challenging operating conditions

AbstractThe construction joint venture NouvLR is constructing the new light rail network in Montreal. One of the major challenges is the advance below the existing runways and taxiways of the Montreal‐Trudeau International airport and the construction of the subway station below the airport. The airport must remain under operation during the construction, and very tight requirements have been imposed regarding the tolerable surface settlements and availability of the monitoring data. The regulations regarding operations of the airport and possible presence of foreign objects in vicinity of its runways and taxiways represent an additional challenge, requiring usage of less straightforward monitoring concepts. The works for the installation of the monitoring equipment must be closely coordinated with airport operations and require a reliable schedule.The final geotechnical monitoring design has been performed in close cooperation with the contractor and in tight coordination with the airport authority. In order to allow more straightforward communication, 3D modelling and BIM‐methodology have been used to clearly represent the monitoring equipment, as well as the works required for their installation. The line‐of‐sight considerations in case of tachymeter measurements have been thus directly incorporated and dealt with. The monitoring design has to fulfil the aims of real‐time monitoring of the system behaviour during the construction and the long‐term monitoring of the newly built tunnel, demonstrating the compliance of the structure with the design. The paper is concluded by a summary of ”lessons learned“ regarding the issues of adverse accessibility and a challenging, high risk general environment.

Application of laser scanning technology for structure gauge measurement

AbstractOne of the basic conditions guaranteeing safe and collision-free operation of rail vehicles is maintaining the structure gauge. Verification of the location of rail infrastructure components is carried out using various measurement techniques. This article describes the accuracy of the gauge measurement with the use of a scanning tachymeter. Tachymetric measurements and laser scanning technology were used in the experimental study of the tram loop area. The analysis covered the possibility of using a point cloud to determine geometrical relationships among the track, traction poles and the overhead line. The quality of the laser scanning data in terms of the measurement frequency, the laser beam angle of incidence per object and the average reflection intensity was examined. Performed verification was based on the data from tachymetric measurements. In the tested area, the track consists of straight sections and several circular arcs of small radii and variable curvature. The specific geometry of the track required calculation of the gauge extension parameters depending on the curve radius. In addition, a horizontal track alignment design was prepared. The designed location of the track and extended dimensions of the structure gauge were used to verify the correct spatial position of the current track in relation to the infrastructure elements.

INFLUENCE OF RANGING UNCERTAINTY OF TERRESTRIAL LASER SCANNING ON CHANGE DETECTION IN TOPOGRAPHIC 3D POINT CLOUDS

Abstract. Terrestrial laser scanners are commonly used for remotely sensing natural surfaces into 3D point clouds. Time series of such 3D point clouds can be analysed to gain information of surface changes that are induced by Earth surface shaping processes. The atomic unit in time series analysis is a bitemporal change detection and quantification. This should involve an estimation of the minimum quantifiable change, the Level of Detection, to separate signal from noise, e.g. stemming from the measurement. To enable such an estimation through error propagation, a model of the sensing instrument’s measurement uncertainty is required. In this work, we present an investigation on the ranging component of terrestrial laser scanning on this uncertainty and its influence on 3D distances between point clouds of two epochs. Specifically, we analyse the effects of incidence angle, intensity and range for different object materials, and make additional considerations with respect to waveform information returned by the sensor. We estimate a model for the rangefinder uncertainty of a terrestrial laser scanner and apply it on experimental data. The results show that using a sensor-specific model of ranging uncertainty allows an appropriate estimation of the Level of Detection. At a range of 60 m and a rotational displacement of 10°, this Level of Detection ranges between 0.1 mm to 1 mm for a white and a grey surface and up to 5 mm for a black surface. The completeness of the detection of significant change ranges from 60.2 % (black) to 89.8 % (grey) for the proposed method and from 65.5 % to 88.9 % for the baseline, when compared to tachymeter measurements. The similarity between the results is expected and suggests the validity of error propagation for the derivation of the Level of Detection.

Application of HDS techniques to bridge inspection

This paper analyses the usefulness of high-definition survey (HDS) techniques in structural inspection. To this effect, tachymetric and HDS measurements are conducted on a beam bridge located in Avilés (Asturias, Spain) that had suffered damage from an impact, which led to the loss of part of the constituent material of one of its exterior beams. HDS techniques are primarily aimed at determining one of the edges of the damaged beam and the results are then compared with the tachymetric measures of the same element. The accuracy of HDS techniques is determined by comparison to that of the tachymetric measures. In addition, by combining HDS techniques and computer-aided design (CAD) systems, the material loss produced by the impact to the damaged beam can be quantified. For this purpose, healthy sections of that beam are compared with the damaged ones. The conducted work, despite the complexity and laboriousness of HDS processing, shows the potential of this technique in the inventory of infrastructures as well as in structural inspection through the characterisation of beam deflection and material losses.

The Investigation of the Ardales Cave, Spain – 3D Documentation, Topographic Analyses, and Lighting Simulations based on Terrestrial Laser Scanning

AbstractThis contribution shows the application of terrestrial laser scanning in an integrative approach for the documentation and analyses of the Ardales Cave, southern Spain, which is in many respects an important geoarchaeological site. For the survey of the cave, a combination of the Riegl LMS Z420i laser scanner with a real‐time kinematic global positioning system (RTK‐GPS) from Topcon and further tachymetric measurements were used. The achieved three‐dimensional (3D) model of the cave and the surrounding hill documents the current topography and dimensions of the cave. Additional geoarchaeological data were successfully integrated in a 3D geographical information system (GIS) database and high‐resolution records of a structured‐light scanner were combined with the 3D model of the cave. The 3D model is further used for the estimation of the ceiling thickness that reveals areas for additional entrances. Lighting simulations based on path tracing were conducted for the determination of areas that are reached by natural direct or indirect light. In this case, the weight and size of the instrument was a logistic constraint to reach certain areas and to achieve a complete model of the cave. Overall, the method is feasible for the documentation of this cave and the investigations based on the derived 3D models. Copyright © 2015 John Wiley & Sons, Ltd.

Reconstructing fluvial, lacustrine and aeolian process dynamics in Western Mongolia

At the eastern rim of the Mongol Els, the largest dune field of Mongolia, the rivers Shurgyn Gol and Zavkhan Gol converge and form a flood plain at the dune front. Interfingering fluvio-lacustrine and aeolian sediments are found in different spatial contexts in the area. By investigating the chronostratigraphy of these sediments, valuable information can be derived for the reconstruction of geomorphological processes and forcing-response relations in the past. The flood plain is locally covered by lacustrine sediments representing an Early to Mid-Holocene lake only a few meters deep. This can be deduced from several 14C and OSL dates and tachymetric measurements, and is in accordance with high stands of other lakes in Western Mongolia. Relics of a lake sediment on top of an old dune 17 m above the floodplain have been OSL-dated to 114 10, 119 10, and 127 10 ka, indicative of an Eemian age. At that time a large and deep lake must have covered the plain. There is evidence of two causes of lake formation: increased discharge of the rivers due to increased precipitation in the mountainous catchment areas and, at the same time, ponding of the rivers by strong dune advance due to persistingly arid conditions in the foreland. The transition phase to more humid con ditions, together with a relatively high humidity gradient, appears to have been morphodynamically very efficient. No evidence could be found of the assumed modern climate change. Recent reduction of the sparse grass cover of the region reflects overgrazing, and the reduction of river discharge is due to water-consuming irrigation.

Assessment of the influence of flying altitude and scan angle on biophysical vegetation products derived from airborne laser scanning

Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical vegetation properties such as tree height, crown width, fractional cover and leaf area index (LAI). It is expected that the conditions of data acquisition, such as viewing geometry and sensor configuration influence the value of these parameters. In order to gain knowledge about these different conditions, we test for the sensitivity of vegetation products for viewing geometry, namely flying altitude and scanning (incidence) angle. Based on two methodologies for single tree extraction and derivation of fractional cover and LAI previously developed and published by our group, we evaluate how these variables change with either flying altitude or scanning angle. These are the two parameters which often need to be optimized towards the best compromise between point density and area covered with a single flight line, in order to reduce acquisition costs. Our test‐site in the Swiss National Park was sampled with two nominal flying altitudes, 500 and 900 m above ground. Incidence angle and local incidence angle were computed based on the digital terrain model using a simple backward geocoding procedure. We divided the raw laser returns into several different incident angle classes based on the flight path data; the TopoSys Falcon II system used in this study has a maximum scan angle of ±7.15°. We compared the derived biophysical properties from each of these classes with field measurements based on tachymeter measurements and hemispherical photographs, which were geolocated using differential GPS. It was found that with increasing flying height the well‐known underestimation of tree height increases. A similar behaviour can be observed for fractional cover; its respective values decrease with higher flying height. The minimum distance between first and last echo increases from 1.2 metres for 500 m AGL to more than 3 metres for 900 m AGL, which does alter return statistics. The behaviour for incidence angles is not so evident, probably due to the small scanning angle of the system used. fCover seems to be most affected by incidence angles, with significantly higher differences for locations further away from nadir. As expected, incidence angle appears to be of higher importance for vegetation density parameters than local incidence angle.

River Dike Deformation Measurement With Airborne SAR

Due to extremely high water levels or extremely dry periods, river dikes can deform and eventually burst. The deformation can be measured with repeat-pass synthetic aperture radar (SAR) interferometry. Two flights (5.5-day interval) with an airborne C-band SAR were carried out to verify a particular case. During the second flight, the dike was deliberately deformed. Interferogram analyses showed that the dike deformed by at most 2 mm on this day, in approximate agreement with tachymeter measurements. Corner reflector analysis showed a deformation of about 1 cm during the 5.5-day interval, in good agreement with ground measurements.