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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resolution of each pixel. For this zenithal portion, a GSD value of 1-2 cm is optimal in most cases. The subsequent segment is a manually piloted flight focusing on bridge components (including piers, abutments, and tdeck).

Figure 1 UAS Aerial Photogrammetry workflow.

This part entails capturing images from various perspectives with a sub-centimetre GSD, yielding a point cloud denser than that produced with ground-based laser scanning. Both sets of images are then concurrently processed through a standard Structure from Motion (SfM) technique using specialized photogrammetric software. 2.2. Drone Preparation The drone and relative camera payload will then be assembled and configured. The camera functionality must be checked and the shooting parameters set up (resolution, exposure, time shooting sequence for the manual portion of the flight, selecting the shooting interval to 3s which is the manufacturer recommended method due to optimization of algorithm used). When the final UAS assembly is completed, the complete system check includes: • Enabling of the Real-Time Kinematics (RTK) function and verifying that the RTK network is sync with the UAS. • Determining the automated flight plan sequence to cover the target and allow for the oblique angle (45°) sequence. • Inserting and comparing the aircraft altitude height with the precomputed one to achieve the desired GSD and oblique GSD values (dependent on type of sensor used, field of view (FOV), and focal length parameters). • Verifying the flight plan trajectory that will affect the photo shooting sequence. 2.3. Supporting measuring equipment Before starting flight operations for precise measurement of the ground target, it is necessary to record their Global Navigation Satellite System (GNSS) location with a sub-cm accuracy. This operational data collection ensures that in case of the UAS RTK sensor partial or total failure (data link lost), it would still be possible to proceed with data acquisition and later use the collected target data to accurately georeference the 3D model and the orthomosaic, or any other kind of desired deliverables. 2.4. Calibration and testing The RTK system is a positioning service offered by many Italian regions. It operates through a network of stations providing real-time differential corrections via the NTRIP protocol (Networked Transport of RTCM via Internet Protocol). This service enables high-precision GPS localization for geolocation and mapping applications. To access the RTK service, users must register on dedicated regional portals to obtain a username, password, IP address, and access port (available 24/7, year-round). These details are crucial for connecting to the network and receiving real time corrections. A test flight should be performed to ensure the drone operates correctly, as well as the calibration of any additional sensors (like LiDAR or thermal sensors).