# Unmanned Aerial Vehicle Depth Inversion to Monitor River-Mouth Bar Dynamics
## 2 Video Acquisition and Rectification
### Image Acquisition and Rectification  
This study employed a DJI Phantom 4, a commercially available small UAV, for the video observations. The UAV camera has a 4K resolution (3860 \(\times\) 2160 pixels) and a video framerate of 29.97 fps. We recorded oblique aerial videos of each test area by hovering the UAV in a stationary position at an elevation of 100-150 m above mean sea level. As shown by an example image in Figure 3a, the horizontal camera angle was set obliquely to the shoreline in a similar way as previous studies [18; 19]. The recorded image contains ground control points (GCPs) on land and captured swell patterns in high image contrast. Table 1 summarizes general information about the field tests. Cases 1 and 2 were conducted in our previous study [19]. We carried out Cases 3 and 4 at the river mouth (A3) in September 2019 before and after a powerful typhoon hit, i.e., Typhoon Faxai, which approached the area on 8 September, accompanied by strong winds, high waves, and heavy rain [20] and significantly impacted coastal and river-mouth bathymetry. During the peak period of the typhoon, the significant wave height reached 6.0 m at the same station (Figure 2). Therefore, we explored the morphological impact on the river-mouth bar with Cases 3 and 4. The significant wave height and period for each case observed with the bottom pressure sensor  
Figure 1: Study area: (**a**) location of the Suruga Bay, (**b**) location of the Suruga Coast and (**c**) Oi River, and three test areas, A1, A2, and A3 on the Suruga Coast. at Omaezaki Station are shown in Table 1. The table also includes tidal water level at each observation time, which we used for the tide correction of the estimated bathymetry data. Figure 3: An example frame of the video and ground control points (GCPs) indicated by red circles (Case 4): (**a**) a video frame by UAV and (**b**,**c**) enlarged views of ground control points. Figure 2: Wave and tide observation data at Omaezaki Station during the test period (October 2018 to September 2019). (**a**) Significant wave height (SWH), (**b**) Significant wave period (SWP) and (**c**) Tidal water level (TWL). The significant wave height and period were obtained from the Nationwide Ocean Wave information network for Ports and Harbors (NOWPHAS), while the tidal water levels were observed by Japan Meteorological Agency (JMA). Before depth inversion, we converted each recorded video into orthogonal video imagery in the same way as [19]. Even when operated in the hover mode, the UAV occasionally moved in horizontal and vertical directions due to strong winds. First, we estimated the camera position and angle by geospatial information analysis using the GCPs following [21]. Some ground objects, such as the corners of coastal structures, were utilized as GCPs (Figure 3b). Additionally, blue-coloured sheets were laid out on sandy beaches when distinctive objects were not available (Figure 3c). The geographical coordinates of the GCPs were measured by real-time kinematic (RTK) GPS during the field observations. The corresponding image coordinates were automatically detected in each video frame using the image template matching algorithm. Then, we created orthogonal video imagery by projecting the original video frames onto a horizontal target area. The rectified image covers a target area of 600 m in the cross-shore direction and 1000 m in the alongshore direction. The sampling rate was reduced from 29.97 to 6 fps (\(\Delta t\) = 0.167 s) to decrease the computational cost in the following depth-inversion process. Table 2 summarizes the video length and camera parameters estimated from the geospatial analysis. The UAV was hovered in a stationary position at an elevation of 100-150 m above mean sea level. The elevation was set within the legal maximum altitude of Japan, 150 m above ground level. Wave propagation in each coastal area was filmed with a horizontal angle of 30-55\({}^{\circ}\) seaward from the shoreline and an elevation angle of 60-80\({}^{\circ}\) upwards of the horizontal plane.