Sensing Heathland Vegetation Structure from Unmanned Aircraft System Laser Scanner: Comparing Sensors and Flying Heights

Low-cost lidar sensors mounted on unmanned aircraft systems (UAS) can be applied for the acquisition of small-scale forestry applications providing many advantages over airborne laser scanning (ALS), such as flexibility, low flight altitude and small laser footprint. Compared to 3D data generated from dense image matching using photogrammetry, lidar has the advantage of penetration through the canopy gaps, resulting in a better representation of the vertical structure of the vegetation. We analyse the effect of different flight altitudes on the penetration rate of heathland vegetation in the Blue Mountains, Australia using a Phoenix system based on a Velodyne Puck 16 scanner and a GreenValley LiAir X3-H system based on a Livox scanner. The different sensors achieve quite different performances, especially for the mid-vegetation layer between the canopy and the ground layer. Representation of this layer is especially important when investigating fuel availability for bushfire analyses. In this layer, the LiAir system achieves a sufficient picture at an altitude of 65 m above ground, whereas the Phoenix system needs to be flown as low as 40 m to get a comparable result.