Effects of flight altitude on tree height estimation using airborne laser scanning

High-density airborne laser scanner data has been previously shown to provide great opportunities for individual tree detection and measurement of variables characterizing the detected trees. This paper evaluates the effect of laser flight altitude on the tree height estimation at individual tree level in the boreal forest area mainly consisting of Norway spruce, Scots pine and birch. The test area (0.5 km by 2 km) was flown at three altitudes (400 m, 800 m and 1500 m) with TopoSys Falcon scanner in spring 2003. Field inventory was carried out on 33 sample plots (about 30 m x 30 m) in the test area during summer 2001. Trees with diameter at breast height larger than 5 cm were measured. Position, height and species of the trees were recorded. 13 plots were dominated by spruce (>50 %), 7 plots by pine, 6 plots by deciduous trees and the rest were mixed forests. Laser point clouds in the circle of varying radius around the trees were used to extract information about spatial distribution of tree crown and height without delineation of individual trees first. Evaluations of estimation errors due to flight altitudes, including beam size and pulse density, were performed for different tree species. The results indicate, in general, that tree height estimation accuracy and number of detectable trees decreases with the increase in flight height. Point density has more influence on tree height estimation than footprint size. Birch is less affected than coniferous trees by the change in the flight altitude.