PSI - Issue 62

Antonio Di Pietro et al. / Procedia Structural Integrity 62 (2024) 755–762 Antonio Di Pietro et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2.5. Execution of the survey flights The best series of flights must be defined to extensively cover the structure, collecting a proper amount of data in order to achieve the desired level of accuracy. A preliminary safety and reconnaissance flight will be performed to verify the obstacle height. Then the acquisition flight sequence will be set. For cataloging purposes and to facilitate the data modeling elaboration process, the first 3 flights would be dedicated to nadiral data acquisition: i) the automated sequence collects photos of the target with a depression angle of -90° and the automated flight plan is in oblique mode at an altitude of 35 m; ii) the second flight plan has similar trajectories at an altitude of 45 m to ensure proper surface coverage; iii) the third flight plan is taken at an altitude of 30 m, and with the trajectory parallel to the longitudinal axis of the bridge in order to capture the upper side of the structure with a different GSD. The altitude selected are functions of the optimization of the GSD factor, lighting condition, and best camera shooting parameters to maximize precision and decrease possible measurements error. The reference plane for 2D/3D reconstruction is chosen as the bridge pavement, primarily because it serves as a known reference altitude and plane, essential for flight techniques, operation, and mission planning in auto mode. For better accuracy and to ensure that the acquired data are accurate and sufficient to generate the orthomosaic, the collected data can be downloaded and then processed with SfM software, in low density mode to determine the quality and distribution of the point cloud. It must be verified whether the flight trajectory is accurate and covers the target area in a detailed and precise manner, and if data are satisfactory. Whenever possible, it is important to capture shoots that contain as much as possible the target of the measured Ground Control Points (GCP) to facilitate, later in the processing phase, the software algorithm in the triangle creation and stitching operations of the pictures database. Figure 2 shows the trajectory of manual flight executed with a Mavic 3M, with RTK datalink correction to capture the lateral details of the bridge. 2.6. Acquisition, processing, analysis, and interpretation of data Aerial photos must be collected following the sequence established in the flight plan. It is fundamental to ensure that images are of high quality and cover the entire area of interest to produce an accurate model. The presence of missing data in the point cloud requires a repetition of data acquisition, increasing the cost and time required for completion. The process requires the mapping of the area of interest as it determines the survey coverage and ensures that all relevant elements are included. The images acquired will be processed with a photogrammetry software to create Digital Terrain Models (DTM), Digital Surface Models (DSM), orthophotos, etc. An accurate analysis of the produced models and maps allows to detect features or changes in the landscape and the use of data for specific applications (such as urban planning, precision agriculture, or environmental monitoring).

Figure 2. Beroide Bridge: north side view, and detail of the underpass (left). UAS manual Flight technique for under pass data collection (right).

3. Collecting aerial data In aerial photogrammetry, the meticulous selection of targets (considering their material, design, size, and color) is pivotal to the survey specific requirements, guaranteeing precision and optimal outcomes. Georeferencing refers to the process of positioning a digital image in the real world. In aerial photogrammetry, targets provide a foundation for georeferencing, allowing software to calibrate, scale, and align the aerial images. By having spatially accurate reference points, the resulting map or model is dimensionally correct. PVC is commonly used for making targets because of its lightweight and flexible nature. It is easy to roll, making it portable. Reinforced eyelets enable the PVC targets to be anchored firmly to the ground, ensuring they remain in position. Wooden targets, when painted, offer a