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
* attila.berenyi@mail.bme.hu 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
* tlovas@mail.bme.hu 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
Marc H 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 | M a r cH 2 009
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
claim confirmed in lab tests The main advantage of laser scanning
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
Marc H 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. and Szöcs, K., 2008. Terrestrial laser
figure. scanning can allow measurements not scanning in deformation measurements of structures.
possible by traditional methods. international archives of Photogrammetry and remote
Scanning PoTenTial However, since laser scanning cannot Sensing, 37 (B5), 527–531.
Based on the presented results, be evaluated on the same accuracy lovas, T., Barsi, Á., Polgár, a., Kibédy, Z., Berényi, a., Detreköi,
terrestrial laser scanning has level as the traditional high-precision Á. and Dunai, l., 2008. Potential of terrestrial laser scanning
demonstrated its potential in equipment, it should only be deformation measurement of structures. in: Proceedings of
deformation and load test considered as a useful additional aSPrS annual conference, Portland, USa; 28 april – 2 May; p.
measurements. The laser-scanned measurement method. 10.
point cloud holds information about lovas, T., Berényi, a., Barsi, Á., Dunai, l. 2009. Supporting the
the whole visible part of the structure. acknoWleDgeMenTS load Test Measurement of the Megyeri Bridge with Terrestrial
It enables the displacement and The authors would like to thank laserscanning (a Megyeri híd terhelésvizsgálatának
deformations to be measured during Burken Ltd for the scanners and their támogatása földi lézerszkenneléssel) [in Hungarian], Geodézia
the post-processing, without the use valuable help with data acquisition és Kartográfia, Budapest, Vol. lXi, no. 1, pp. 20-26.
of previously highlighted control and processing. This paper was
points. Analysing the structure’s supported by the János Bolyai
displacements and distortions in 3D Research Scholarship of the
provides reasonable information for Hungarian Academy of Sciences.
No1182
Marc H 2009 | inTErnaTional | 21