2 research outputs found
Self-calibration of wireless cameras with restricted degrees of freedom
This paper presents an approach for the automatic calibration of low-cost cameras which are assumed to be restricted in their freedom of movement to either pan or tilt movements. Camera parameters, including focal length, principal point, lens distortion parameter and the angle and axis of rotation, can be recovered from a minimum set of two images of the camera, provided that the axis of rotation between the two images goes through the camera’s optical center and is parallel to either the vertical (panning) or horizontal (tilting) axis of the image. Previous methods for auto-calibration of cameras based on pure rotations fail to work in these two degenerate cases. In addition, our approach includes a modified RANdom SAmple Consensus (RANSAC) algorithm, as well as improved integration of the radial distortion coefficient in the computation of inter-image homographies. We show that these modifications are able to increase the overall efficiency, reliability and accuracy of the homography computation and calibration procedure using both synthetic and real image sequence
Condition assessment of bridge structures by damage localisation based on the DAD-method and close-range UAV photogrammetry
The provided dissertation presents a so-called “Deformation Area Difference (DAD)” method
for condition assessment of existing bridges, especially for the detection of stiffness-reducing
damages. The method is based on the one hand on conventional static load deflection
experiments and on the other hand on a high-precision measurement of the structural deflection.
The experimental load on the bridge should be generated within the serviceability limit state in
order to enable a non-destructive inspection.
In the course of the laboratory tests, the most innovative measuring techniques were applied,
whereby the photogrammetry has delivered promising results. With the help of additional
studies on the influences of camera quality and calibration, the measuring precision of
photogrammetry could be brought to its limits.
Both the theoretical investigations and the laboratory tests showed the successful use of the
DAD method for the identification of local damages. Therefore, the first in-situ experiment was
carried out on a single-span, prestressed bridge in Luxembourg. The knowledge gained from
this was combined with statistical investigations based on finite element calculations and
artificially generated measurement noise effect in order to determine the application limits, such
as the achievable measurement precision, identifiable degree of damage, required number of
measurement repetitions, influence of the damage position, optimal size of the structural
deformation, etc.
The development of the DAD method ready for application usefully supplements the state of
the art and contributes to the reliable assessment of the bridge condition