Impacts of UAV–LiDAR flight altitude and forest canopy on the estimation accuracy of understory terrain

Unmanned aerial vehicle light detection and ranging (UAV–LiDAR) is a new method for collecting understory terrain data. The high estimation accuracy of understory terrain is crucial for accurate tree height measurement and forest resource surveys. The UAV–LiDAR flight altitude and forest canopy cover significantly impact the accuracy of understory terrain estimation. However, since no research examined their combined effects, we aimed to investigate this relationship. This will help optimize UAV–LiDAR flight parameters for understory terrain estimation and forest surveys across various canopy cover. This study analyzed the impacts of three flight altitudes and three canopy cover on the estimation accuracy of understory terrain. The results showed that when canopy cover exceeded a specific value, UAV–LiDAR flight altitudes significantly affected understory terrain estimation. Given a forest canopy cover, the reduction in ground point coverage increased significantly as the flight altitude increased; given a flight altitude, the higher the canopy cover, the more significant the reduction in ground point coverage. In forests with a canopy cover≥0.9, there were substantial differences in the accuracies of understory digital elevation models (DEMs) generated using UAV–LiDAR at different flight altitudes. For forests with a canopy cover < 0.9, the mean absolute error (MAE) of understory DEMs from UAV–LiDAR at different flight altitudes was ≤ 0.17 m and the root mean square error (RMSE) was ≤ 0.24 m. However, for forests with a canopy cover≥0.9, the UAV–LiDAR flight altitude significantly affected the accuracy of understory DEMs. At the same flight altitude, the MAE and RMSE of the estimated elevation for forests with a canopy cover≥0.9 were approximately twice those of the estimated elevation for forests with a canopy cover < 0.9. In forests with low canopy cover, it is possible to improve data collection efficiency by selecting a higher flight altitude. However, UAV–LiDAR flight altitudes significantly affected understory terrain estimation in forests with high canopy cover, it is essential to adopt terrain-following flight modes, reduce flight altitudes, and maintain a consistent flight altitude during long-term monitoring in high canopy cover forests.