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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matte finish, preventing glare under direct sunlight, which may degrade the photo quality. For more controlled environments like indoor studios, paper targets can be used, though they are fragile and often considered expensive. The most versatile and easily identifiable symbol is the “X”, consisting of four triangles, with a central intersection point. This design allows many photogrammetry software with recognition algorithms to automatically detect and pin these targets. Regarding the color, the contrast between two colors makes a target easily detectable. The most efficient combinations are red-black, yellow-black, and black-white, with the latter providing the highest contrast. Larger targets are more discernible from greater altitudes. Although portability becomes an issue with size, for drone flights over 80 m above ground, targets should ideally be at least 1 m x 1 m. Labels on targets, especially on the exterior part, can be invaluable, offering an easy way to relate photographic data to GPS readings, and providing a structured order for processing. The purpose of the survey dictates the choice of targets. For photogrammetry of small to medium sized objects under controlled lighting, smaller targets may suffice. However, for extensive aerial surveys, especially when using a camera with limited performance, the size of the target becomes paramount. 3.1. Placement of Targets Targets (GCPs) are used to accurately georeference the images captured by the UAS. When using GCP for aerial photography, an important aspect is the target placement strategy (i.e., number, position, dimensions, coloring, and documentation) and the technique used to identify targets. The number of targets significantly affects georeferencing accuracy, with more targets generally providing better results. As a general practical rule, for a bridge spanning 200 m, a recommended minimum quantity is around 10 to 12 targets. Targets should be strategically placed near the structure of interest (Figure 3) and should coincide with easily detectable and measurable points on the bridge in order to map images with the structure.

Figure 3. Example of target placement and typology.

To aid SfM software in automatic detection of targets, one option is to use aerial spray (washable and eco-friendly) that can be used in lieu of the standard target to indicate a GCP. In addition, the placement of targets should be evenly across the bridge for comprehensive coverage and redundancy, aiding error detection and correction during georeferencing. The identification of targets through a numerical (or alphabetical) sequence of labels, along with georeferencing data, are essential for scale bar purposes. In cases where GNSS signal from a station is unavailable, the recorded sequence and a visual record of the targets can be valuable for georeferencing the 3D model. The combination of georeferencing data and systematic target identification is a reliable fallback option when GNSS signal is weak or lost, ensuring the model accuracy is maintained. 3.2. Initialization of the GNSS/RTK station In the realm of aerial surveying, RTK technology provides surveyors and industry specialists with cm-level positioning accuracy. Different from traditional satellite systems, real-time kinematics greatly enhances accuracy, by correcting errors instantaneously during flight. A RTK system consists of: (i) a Base Station positioned at a fixed point on the ground to compare its GPS location with that of the drone, ensuring accuracy; (ii) a RTK receiver integrated into the drone allowing communication with both the controller and satellites; (iii) a remote control that guides the drone and monitors coordinates in real-time; (iv) the availability of satellites. Despite RTK's potential, this technology requires the continuous availability of connection with the controller or satellites. In such scenarios, real-time data