BY ATTILA BERÉNYI AND TaMÁS LOVAS, BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS, HUNGARY FEATURE ACQUIRING 3D SPATIAL DATA OF BRIDGES Laser Scanning in Deformation Measurements State-of-the-art geodetic and remote Terrestrial laser scanning is a state-of- target, allowing the distance between sensing techniques can prove their the-art remote sensing technology the scanner and object to be calculated. potential through particular that can rapidly acquire accurate three- For accurate point location in space, engineering applications. Here we dimensional (3D) spatial data. The the reflection angle is also recorded. discuss load test measurements of primary engineering applications bridges over the Danube. Prior to the include architectural surveying, LAB MEASUREMENts particular bridge surveying projects, mining volume analysis and measuring In order to validate the accuracy of the accuracy analysis was carried out in complex mechanical systems for terrestrial laser scanners used in the the laboratory. We describe the modelling. We discuss the potential of surveying projects, laboratory shortcomings and limitations of remote laser scanning in engineering geodesy, measurements were carried out. To sensing and propose the joint i.e. displacement and deformation investigate the root-mean-square error application of traditional and remote measurement. The technological (RMSE) of the laser scanner ranging, sensing technologies. We also provide capabilities are validated by laboratory the deformation of a steel plate was recommendations for further measurements including comparative measured. The displacements were also applications of laser scanning in Figure 1, Case study analysis. The outdoor potential of laser measured by high-precision digital deformation measurement of locations. scanning is shown by load test calliper. The results confirmed the structures. measurements of two Danube bridges. scanner manufacturer’s claim of ±5mm The results demonstrate the potential ranging accuracy. In the second phase of terrestrial laser scanning in such of the laboratory tests, laser scanning Attila Berényi is a PhD student at the engineering projects. Department of Photogrammetry and Geoinformatics, Budapest University of THE TECHNOLOGY Technology and Economics, Hungary. The result of a laser scanning * [email protected] measurement is an accurate 3D point cloud describing the surveyed object. Megyeri bridge The range of the applied laser scanners Tamás Lovas is an assistant professor at is currently 2–800m; they can Budapest the Department of Photogrammetry and therefore be applied indoors (e.g. Geoinformatics, Budapest University of laboratory test of portal frame) and on Technology and Economics, Hungary. site (e.g. load test of a bridge). The * [email protected] concept of laser scanning is based on measuring the time for an emitted Pentele bridge 20 km beam of light to be reflected from the 10 mi MARCH 2009 | INteRNATIONAL | 17 of the load test of a Lindab small PENTELE BRIDGE Figure 2, Pentele computed maximum vertical building systems (SBS) portal frame The Pentele Bridge consists of three Bridge and the laser displacement is about 35cm with both was carried out. The displacement of main parts: two parts over the flood scanning station. methods in the fourth load case. The particular points of the structure was plain and the main part over the river reason for the oscillation of the curves measured by inductive transducers, Danube. The focus of the study is Figure 3, Megyeri derived from laser-scanned data is the while the stresses were measured by exclusively on the middle part of the Bridge. reduced point density. The trend of strain gauges. The primary objective bridge, which is over the river. It is a the curves and the displacement was to measure each of the load cases basket-handle tied-arch bridge with a values validate the laser scanning by laser scanner and to derive the span of 307.8m (the world record in measurements. particular displacements. this category) and height of 48m. Additionally, comparing the results Since the load cases lasted for only a MEGYERI BRIDGE with values obtained from traditional short period of time (20–30 minutes, This cable-stayed bridge is the longest high-precision equipment enabled the minimum time needed for the river bridge in Hungary (1,861m) and the accuracy of the laser scanning geodetic measurements) and cannot consists of five bridges (nine bridge to be analysed and its potential for be repeated, only one laser scanning structures). We focused on the such projects to be evaluated. station was selected (Figure 2) and the largest bridge structure that spans As well as comparing the 3D models scanning resolution was reduced. The Table 1, the Danube with a length of 591m of each state, direct measurements displacements of particular points of Parameters of laser (Figure 3). In this load test, two of the point clouds were also two structural features (the northern scanning. terrestrial laser scanners validated. arch and the bottom part of the girder) simultaneously scanned each load BRIDGE TESTS Pest (Riegl Z420i) Island (Riegl Z390i) During the load tests of the bridges, Scan Measurement Resolution No. of Measurement Resolution No. of the vertical movements of the bridge time (°) points time (°) points deck and the stresses (both in discrete Overview 1min 30sec 0.20 716,604 1min 29sec 0.20 713,216 points) were measured. The vertical Detailed 21min 4sec 0.03 5,756,028 21min 14sec 0.03 6,407,291 displacements of the deck (and the main girder) were measured by high-precision levelling. The 3D close to the scanner were therefore case. A Riegl Z420i was deployed at movements of predefined points of obtained. The structural displacements the Pest riverbank (Figure 4, Pest) the structure were determined by derived from the laser-scanned data and a Riegl Z390i was located on the total stations, while the stresses sets are very similar to those derived side of Szentendrei Island (Figure 4, were measured by strain gauges. by traditional techniques. The island). During the load test, the 18 | INteRNATIONAL | MARCH 2009 FEATURE Figure 4, False- displacement values are more colour point cloud correlated on the side of the bridge (Megyeri bridge) with from which the scanning laser scanning measurements were acquired. stations. Considering all the circumstances of the measurement (dark environment, movements during the measurements and the size of displacements), the ±5mm accuracy of the laser scanner can be considered adequate for describing the displacements of the deck. Note that due to the lower point density, the investigation of the middle segment of the bridge is omitted from the evaluation. Manufacturer’s accuracy CABLE MOVEMENTS The main advantage of laser scanning claim confirmed in lab tests can be observed in measurements that cannot be executed by traditional methods or would make the evaluation unaffordable. The point laser scanners operated with the these elements was investigated. In density of the laser-scanned point parameters listed in Table 1. The the evaluation procedure, the results cloud enables the modelling of the resolution determining the number of the high-precision levelling (RMSE cables and hence the evaluation of of points and point density was set is less than 1mm) were used as a their movements. Such analyses are by the time required by the high- reference. The clear correlation of the not supported by traditional geodetic precision levelling in each load case. laser scanning results compared to measurement. The displacements those from the levelling is depicted in (with respect to the unloaded state) of EVALUATING RESULTS Figure 5. the northern cables of the Pest The result of laser scanning is a raw riverbank pylon are depicted in 3D point cloud. Geometric elements APPLICATION ISSUES Figure 5, Figure 6. It can clearly be seen that can be fitted during the post- The reasons for the differences at Aggregated results. the greatest displacements occur at processing and analysis, and the particular points include the the longest cables. These cables are planar or spatial model of the object following: Figure 6, Cable fixed close to the pier supports and can therefore be generated. Because - Displacements of discrete points displacements. are unable to share the load with the of the great number of points and were measured during the levelling, the high point density, the while the displacements of the visualisation of the raw point clouds fitted lines and arcs were measured enables basic displacement and in the case of laser scanning. deformation tendencies to be - The measurements were made from defined. To exploit the full potential the deck during the levelling. and high accuracy of the technology, However, from the laser scanning measurements of the point cloud stations, the lower part of the were made. No predefined discrete bridge is seen from below, and lines points were captured during laser and arcs are fitted onto the cross scanning, but a particular segment of structures (which move with the the space including all objects in the deck). range of the scanner was measured. - The bridge also moved during the The identification of particular points load cases. That means the laser in the case of displacement is scanner did not capture a snapshot, extremely difficult. To overcome this but the point cloud also describes problem, planar or spatial objects the (minor) displacements that were fitted to the point cloud (e.g. occurred during the load case. lines or cylinders to the cables), and The effect described in the latter point the movement and deformation of can be observed in Figure 5: the MARCH 2009 | INteRNATIONAL | 19 FEATURE deck. Due to the A-shape of the engineers in the investigation of pylons, the cables are not located in a structural behaviour. The evaluation FURthER READING plane. The spatial movements of the of the displacements of the cables and Lovas, T., Barsi, Á., Detreköi, Á., Dunai, L., Csák, Z., Polgár, A., cables are therefore represented in the pylons clearly demonstrates how laser Berényi, A., Kibédy, Z.