PSI - Issue 54

Francisco Afonso et al. / Procedia Structural Integrity 54 (2024) 553–560 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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surface of three different objects were reconstructed. Another method involves multiple cameras, such as the system described by Bertin et al. (2013) utilizing an inexpensive stereo photogrammetric system consisting of digital consumer cameras to measure a gravel bed in different conditions, including underwater. Baqersad et al. (2017) also described a photogrammetric stereo system which uses a triangulation technique to identify targets or features in the object’s surface, whose positions are then tracked throughout the images acquired. In the case of this paper, a photogrammetric system that could be attached to a train’s lateral window w as envisioned, since it would allow it to have a direct line of sight to the retaining walls as the train passes by. Although not common, to obtain the highest amount of information as possible and decrease occlusion points, a stereo system was designed, where two cameras would be positioned at the same level and mildly angled such that their axis converge and their FOV overlaps, this way, two slightly different perspectives of the same retaining wall can be acquired with a single acquisition. The obtained images are then processed in Pix4Dmapper, resulting in a point cloud. A representation of this system can be found in Fig. 1.

Fig. 1. Photogrammetric system representation.

3. System components It was important to select a camera with an adequate resolution and frame rate to enable the capture of a good amount of data, with good quality, especially considering that the train will be moving. To simplify the system, a USB3 connection between the computer and the cameras is preferred. Since the system will have to be implemented in a train, close to the retaining walls, a lens with a small focal length would allow the capture of a bigger area of the wall. Additionally, an adjustable aperture allows precise adjustment of the amount of light to reach the sensor, which will potentially have a big impact in the final image quality. With this in mind, two Pixelink PL-D725CU-T cameras were used with Kowa LM8HC lenses. The Pixelink cameras are able to capture images at 5 megapixels at a maximum of 75 fps, they have a one-inch image sensor and USB3 connections. On the other hand, the Kowa lenses have a focal length of 8 mm and an adjustable aperture of F1.4 to F16.