Enhancing Efficiency while Maintaining Accuracy in Repeated Coastal Drone Monitoring through the Use of Fixed Structures as Ground Control Points

Accurate coastal terrain measurement is essential for effective coastal management. While drones equipped with real-time kinematic (RTK) receivers have improved the efficiency of beach surveying, laborintensive ground control point (GCP) measurements are still required to ensure reliability. This study proposes using fixed GCPs (i.e., stable structures) to correct and validate topographic data acquired by both a conventional drone and an RTK drone and evaluates how different GCP configurations affect measurement accuracy. Experiments were conducted at Jangsa Beach using a conventional drone and Byeonsan Beach using an RTK drone. The analysis revealed a vertical discrepancy of approximately 23.5 m between ellipsoidal height and geoid height using the RTK drone. Even after geoid correction, an additional offset of about 0.125 cm persisted due to discrepancies between camera focal plane positioning and sensor alignment. However, when fixed GCPs were uniformly distributed across the beach, a DEM accuracy of around 4 cm, comparable to using only mobile GCPs, was achieved. In contrast, locally placed GCPs or configurations relying solely on mobile GCPs resulted in improved accuracy only in specific regions (e.g., the central area), while substantial errors remained in boundary regions. This issue was more pronounced with conventional drones, which have lower sensor-based positioning accuracy than RTK drones. Notably, RTK drones achieved an accuracy within 0.05 m even when using only three or more fixed GCPs, demonstrating that minimal GCP placement can still ensure reliable results. In contrast, conventional drones required denser GCP placement or additional mobile GCPs in underconstrained areas to maintain DEM accuracy. These findings confirm that integrating stable pre-surveyed structures as GCPs can significantly reduce the burden of additional GCP installation while maintaining sufficient accuracy. This approach offers a practical and efficient solution for repeated coastal monitoring, minimizing cost and time while ensuring reliable geospatial data acquisition.