Comparison of Coastal Debris Interpretability Across Different GSD Levels in Drone Imagery

Recent studies have increasingly utilized drones for remote sensing, driven by the widespread distribution of marine debris along coastal areas. When monitoring coastal debris using drones, flight altitude is a critical factor that directly impacts both the quality of image data and the monitoring duration. However, designing monitoring systems based solely on altitude may lead to variations in spatial resolution (GSD) caused by differences in camera specifications across various drone models. Such variations in GSD levels impact the interpretability of debris within the imagery. This study evaluates the interpretability of coastal debris at different GSD levels determined by drone specifications and flight altitudes. Based on prior studies, we collected data at four altitudes by GSD: 18.6 m (GSD: 0.5 cm/pixel), 27.9 m (GSD: 0.75 cm/pixel), 37.2 m (GSD: 1.0 cm/pixel), and 46.5 m (GSD: 1.25 cm/pixel). Coastal debris types were categorized into eight classes, defined based on the top 10 most frequently identified debris types over a four-year period in Korea. We also assessed the quality and interpretability of coastal debris data under varying spatial resolutions of drone imagery, with a particular focus on the eight defined categories. Interpretability was assessed based on the National Image Interpretability Rating Scales (NIIRS), developed by Image Intelligence, which defines four interpretability levels: I (Identify), B (Distinguish), D (Detect), and N (Not Detect). The results demonstrated that the interpretability of coastal debris varies depending on debris type, color, and size with changes in GSD. Furthermore, the detectable categories of debris were defined for each GSD level. Through this study, it is expected to support decisions on appropriate GSD settings and monitoring methods for different coastal debris survey objectives and conditions. The findings may also help in developing national policies for managing coastal debris.