## 3 Methodology  
### UAVs for the Monitoring Coast  
Considering the challenges associated with employing fixed-wing or self-assembled UAVs, such as their complexity and user-friendliness issues, we opted for quadcopter commercial integrated UAVs due to their widespread availability and ease of use in the market. We selected three UAVs, namely the DJI Phantom IV Multispectral, the DJI Mavic II Pro, and the DJI Matrice 300 RTK + DJI L1 LiDAR (DJI, Shenzhen, China). Utilizing the imagery captured by these three UAV platforms over the LGTF, we generated DSMs through point cloud data inversion. These DSMs were then compared to detailed ground measurements to assess their accuracy. After that, we performed an entire monitoring of the LGTF using M300-LiDAR, which has the highest scanning efficiency.  
On 9 June 2022, data acquisition was conducted using the three UAVs. Due to the absence of image control points and the variations in flight parameters caused by manual control, DJI GS Pro software ([[https://www.dji.com/cn/ground-station-pro](https://www.dji.com/cn/ground-station-pro)]([https://www.dji.com/cn/ground-station-pro](https://www.dji.com/cn/ground-station-pro)), accessed on 21 March 2024) was employed for flight path planning. Prior to the study, extensive test flights were performed for each system. Considering factors such as flight duration and image quality, a uniform flight altitude of 100 m was set for all UAVs, with both longitudinal and lateral overlap set at 60%. The flight route is depicted in Figure 4B,C.  
It is worth noting that, in light of potential edge errors in the DSM reconstructions caused by insufficient data collection, the flight path's outer regions extended beyond the study area. Furthermore, in conjunction with the subsequent detailed survey area, the flight paths for both groups were optimized and adjusted to ensure that they overlapped more in areas with a higher density of images, thereby enhancing validation accuracy.  
The entire tidal flat monitoring using M300-LiDAR was undertaken on 19 July 2023 (Figure 5) when the spring tide was at its lowest level. In this UAV-based survey, the UAV was configured with specific parameters to ensure optimal data collection. The flight was conducted at an altitude of 100 m above the study area, allowing for a comprehensive aerial perspective. The UAV maintained a consistent speed of 5 km/h, ensuring a controlled and efficient survey process. To capture detailed information, data sampling occurred at regular intervals of 20 m along the flight path. This sampling strategy was implemented to enhance spatial resolution and provide a thorough representation of the targeted area. The entire survey mission had a duration of 40 min, during which the UAV systematically covered the designated region, facilitating a comprehensive and timely data acquisition process. These carefully defined settings were instrumental in achieving accurate, reliable, and timely results for the study. The observed data underwent processing using DJI Terra, an official software ([[https://enterprise.dji.com/cn/dji-terra](https://enterprise.dji.com/cn/dji-terra)]([https://enterprise.dji.com/cn/dji-terra](https://enterprise.dji.com/cn/dji-terra)), accessed on 21 March 2024) that employs photogrammetry and LiDAR technologies to generate 2D and 3D maps of diverse environments.  
Figure 4: **(A)** UAVs employed in this study; (**B**) the planned route of Matrice 300; (**C**) the planned routes of Phantom IV and Maxic II.