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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008
40,
'
101 102 103 104 105 106 107 201 2Ö2 203 204 205
target number
207
Figure 10. Transformed vertical displacements of sphere
centres between different loadings and initial
situation measured by terrestrial laser scanner.
3 0
20
VO
0.0
-30
-4.0-
PI - initial
P2 - initial
R3 - initial
101 102 103 104 105 106 107 201 202 203 204 205 207
target number
Figure 11. Differences between transformed vertical
displacements of TLS and precise levelling for
different loadings.
Mean
residual
(initial-
Pl)
[mm]
Mean
residual
(initial-
P2)
[mm]
Mean
residual
(initial-
P3)
[mm]
Precise levelling
-3.2
-8.1
0.7
TLS
-2.8
-8.9
0.7
A (TLS-levelling)
0.4
-0.8
0.0
Table 3. Mean residuals (vertical displacements) of
deformation analysis for different load situations
detected by precise levelling as well as TLS.
5. DISCUSSION
The results by TLS present relative deformations as deflections
of the cantilever slabs (outer side) of up to 20 mm under a
maximal load of about 100 tons. This deflection range could
only be detected by the area-wide analysis whereas the target
spheres performed relative deformations of up to 6 mm due to
the more central target setup in relation to the bridge girder. For
the detection of absolute deformations of the bridge girder, the
transformations of the TLS data into the reference height
system defined by precise levelling were required.
By comparing the transformed vertical displacements detected
by TLS and the vertical displacements by precise levelling, the
deformations of the bridge girder are within the same range.
Maximum differences between the two measurement methods
are around 3.5 mm. But considering the mean residuals for the
different loading situations, the differences between TLS and
precise levelling are less than 1.0 mm.
Generally, the Felsenau viaduct mainly performed deformations
as settlement and tilting. The deflection of the cantilever slabs
were minor compared to the other deformations.
6. CONCLUSIONS
TLS is a very fast acquisition method and does not require
deployment of any targets on the object. Since the
measurements are carried out touchlessly the performance and
accuracy of the measurements depend on the surface properties
of the object. For scanning road surfaces, black asphalt and
small angles of incident influence the data quality. As for the
Felsenau viaduct, the carriageway could be detected up to a
range of about 20 m from the scanner station.
Regarding deformation monitoring on the Felsenau viaduct,
TLS could replace the area-wide precise levelling. But, the
transformation of TLS data into an absolute height reference
system is essential for the detection of settlements and tilting of
the bridge girder. Hence, for the connection to a height transfer
reference outside of the viaduct precise levelling can not be
omitted.
Our load tests on the Felsenau viaduct have shown the
feasibility of deformation monitoring by TLS. A comparison
with precise levelling allowed assessing the measurement
accuracy and quality of TLS. In general, TLS is suitable for
detecting deformations within the mm-range. But concerning
applications at accuracy level such as the load tests on Felsenau
viaduct, other measurement methods like precise levelling are
indispensable. Therefore, TLS well complements traditional
geodetic measurement methods but cannot replace them
completely.
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