Assessment of Lidar-derived Tree Heights Estimated from Different Flight Altitude Data in Mountainous Forests with Poor Laser Penetration Rates

In this study, the effects of different flight altitudes on tree height estimates with a small-footprint scanning lidar were investigated and assessed in mountainous forests with poor laser penetration rates. The study area was closed-canopy evergreen coniferous plantations dominated by Japanese cedar (Cryptomeria japonica) and hinoki cypress (Chamaecyparis obtusa) in Japan. The stand age ranged from 33 to 100 years and the area was undulating terrain with a variation in elevation ranging from 135 to 391 m above sea level. A total of 33 circular sample plots (0.04 ha) were established and predominant mean tree heights for each plot were calculated using individual tree heights within each sample plot. Data from three different flight altitudes (500 m, 1000 m, and 1500 m) were acquired with Optech ALTM3100 sensor in late summer 2006. The settings of lidar system were paid attention as the laser footprints should cover the targeted area without omission, i.e. the laser spot spacing should be close to footprint diameter in the resultant data. Owing to this idea, we were able to theoretically avoid missing treetops and passing through the canopy gaps just by chance for a given transmitted laser pulse. The results of this study demonstrate that the higher platform altitude would reduce both the penetration rates and the intensities of laser pulses, and affect not only the quality of digital surface model, but also the quality of digital terrain model more significantly in forests with undulating topographies, thus indicating the less accurate estimates of lidarderived tree heights. * Corresponding author. Tomoaki Takahashi; E-mail: [email protected]