Abstract
When applying commercial analysis software to estimate the shallow water bottoms using Unmanned Aerial Vehicle (UAV)- based photogrammetry, the depth of the water as shallow is often underestimated because the refraction of the line of sight at the water surface is not taken into account. This refraction causes significant errors in depth estimation, leading to inaccurate data that can affect various applications, including environmental monitoring and resource management. As a countermeasure, the estimated shallow water depth is corrected by multiplying it by an appropriate coefficient (Water Depth Underestimation Factor). However, some software programs do not disclose their internal triangulation (coordinate estimation) algorithms. Therefore, making it impossible to calculate the Water Depth Underestimation Factor in accordance with the algorithm. In this study, through the use of device to do triangulates underwater points using only photographs whose line of sight is close to the vertical, and conducted experiment for using actual aerial photographs of a small river to verify the effectiveness of this device. The results showed that the Water Depth Underestimation Factor, estimated without using the measured depths, was stable regardless of the 3D location of the underwater points when simple assumptions about the triangulation algorithm were made. Additionally, it was almost consistent with the optimized Water Depth Underestimation Factor derived using the measured depths. This demonstrates that the proposed method is capable of accurately correcting the depth of the shallow water.
