## 1 Introduction
### Coastal Monitoring  
Manual in situ measurements (e.g., via real-time kinematics, RTK) have been considered the most common way to obtain tidal flat topography. However, manual on-site measurements are not the optimal choice. On the one hand, the seasonal variations or responses to extreme weather conditions necessitate the timely acquisition of the latest topography (e.g., within a weekly time frame); on the other hand, when confronting risks in the tidal flat, manual measurements inherently pose potential safety risks. Therefore, unmanned aerial vehicle (UAV) surveys may currently be a more feasible approach, capable of both obtaining topographical data and capturing high-resolution photographs. For industrial or mapping UAVs, the RTK module is an essential component for georeferencing UAV images. To meet a range of research needs, these UAVs might integrate different sensor modules, including super high-resolution cameras, multispectral cameras, LiDAR, thermal imaging cameras, and aerial sensors [17; 18; 19; 20]. UAV systems have gained increasing acceptance in coastal monitoring. The authors of [21] employed a six-axis UAV equipped with a network-based RTK system and a Canon EOS 550D DSLR camera for beach surveys in northeastern Italy. The UAV utilized a structure from motion (SFM) technique, and digital elevation models (DEMs) derived from terrestrial laser scanning were assessed using 15 ground control points (GCPs). The results demonstrated the UAV-acquired data's superior reliability compared to global navigation satellite system (GNSS) data. In a study monitoring long-term changes at Narrabeen Beach, northern Sydney, the authors of [22] utilized a commercial off-the-shelf RTK-global positioning system (GPS) fixed-wing UAV. They exclusively relied on the high-precision RTK positioning system, eliminating the need for ground surveys and GCPs. The study confirmed the reliability of using professional-grade UAVs integrated with the RTK module, even without image control points. By equipping the UAV with a LiDAR module [19], the generated point cloud data exhibited high uniformity (\(\pm\)5-10 cm difference) compared to results obtained through image-based methods. Additionally, they emphasized that, for flat terrains like intertidal zones with extensive UAV coverage, the LiDAR module excelled in penetrating vegetation and capturing original surface points, surpassing the capabilities of other image-based technologies. Researchers have successfully integrated UAVs into their respective fields of study and evaluated the effectiveness of employing small UAVs equipped with various sensors for coastal mapping and study [23]. The existing methods, widely adopted across various industries and supported by comprehensive industry standards, can be categorized into three distinct groups:1. UAVs equipped with an RTK-GPS and visible light cameras are employed for oblique photography to capture.jpg images. Subsequently, 2D-to-3D techniques such as SFM are utilized to generate digital surface models (DSMs) and DEMs [22; 24; 25]. 2. UAVs equipped with multispectral cameras are used to capture.tif or other bitmap format images. Various normalized difference indices are then constructed to enable specialized hyperspectral processing [26]. 3. RTK-enabled UAVs with LiDAR are employed for aerial scans to acquire point cloud files in LDR format, which are subsequently used to construct high-precision DEMs [19]. Figure 2 provides a flowchart of the technical approaches employed in these methods. Figure 2: DSM’s accuracy verification methods for UAV systems. **(A)** Equipment utilized in coastal monitoring; **(B1–B3)** demonstrate the operational procedures for UAV-LiDAR, UAV-camera, and ground-based RTK monitoring, respectively; **(C1,C2)** display the monitoring outcomes derived from these varied methodologies.