3.3. A comparison of helicopter-based VUX-1LR LiDAR with below-canopy UAV photogrammetry and manual measurements

Measurements such as the diameter at breast height (DBH) are useful for estimating the volume of timber in an area of forest and are conventionally measured by hand using a measuring tape or calipers. Remote sensing techniques have great potential to replace the need for manual measurements, allowing rapid, low-cost and accurate assessment of timber stands at large scales. Close-range, Unmanned Aerial Vehicle (UAV) photogrammetry was used to generate a point cloud of the stems from beneath the canopy. In addition to this, a helicopter-mounted Riegl VUX-1LR LIDAR was used to capture a point cloud from above the canopy at 3 altitudes. This study aims to determine which sensing system and flying height provided the best detail in the sub-canopy. To compare the sub-canopy datasets, the measurement of each tree’s DBH was used. The DBH was extracted for 30 trees using data collected at 30m, 60m and 90m altitude by the VUX-1LR LIDAR, as well as from the point cloud created by the sub-canopy UAV. Manual DBH measurements were used as a baseline for comparison and statistical analysis was performed. The greatest correlation of remotely sensed DBH to manually measured DBH came from the below-canopy UAV photogrammetry system, followed by the 60m flying height with the VUX-1LR LIDAR. The 90m flying height did not result in a statistically significant correlation using the methods in this study. Where sub-canopy data is desired, it is recommended to fly at 60m with the VUX-1LR LIDAR in the forest conditions studied. In areas of greater tree density, below-canopy UAV techniques present an interesting solution.