The Effects of Footprint Size and Sampling Density in Airborne Laser Scanning to Extract Individual Trees in Mountainous Terrain

The effects of footprint size and sampling density in airborne laser scanning on extraction of individual trees in a mountainous terrain were investigated. A stand of Japanese cedar was selected for the study. Three flight altitudes of the helicopter above the ground, 300, 600 and 1,200 meters, were used to acquire the data with different footprint sizes. The footprint diameters were approximately 0.3, 0.6 and 1.2 meters respectively. Sampling densities corresponding to three flight altitudes were 24.8, 10.1 and 7.5 points/m. DCM (digital canopy model) for each altitude with 1-meter was generated from three measurements and they were compared to understand the effects of footprint size. Quasi-data of 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64 of sampling density were created from the original first and last pulse data acquired from the measurement at a flight altitude of 300 meters. Individual trees that were extracted from these data were examined in terms of the position and the value of tree height to evaluate the sampling density. The mean values of subtraction of DCM by footprint size of 0.3 meter from DCM by footprint size of 0.6 meter and 1.2 meters were 0.5 meter and 0.9 meter respectively. The rate of extraction of treetops from DCM declined suddenly in case the sampling density was below 3 5 points/m. The difference between mean tree height derived from DCM with high sampling density and one with low sampling density was 0.5 0.6 meters and the height with low sampling density was underestimated.