A rigorous assessment of tree height measurements obtained using airborne lidar and conventional field methods

Tree height is an important variable in forest inventory programs but is typically time-consuming and costly to measure in the field using conventional techniques. Airborne light detection and ranging (lidar) provides individual tree height measurements that are highly correlated with field-derived measurements, but the imprecision of conventional field techniques does not allow for definitive assessments regarding the absolute accuracy of lidar tree height measurements and the relative influence of beam divergence setting (i.e., laser footprint size), species type, and digital terrain model (DTM) error on the accuracy of height measurements. In this study, we developed a methodology for acquiring accurate individual tree height measurements (< 2 cm error) using a total station survey and used these measurements to establish the expected accuracy of lidar- and field-derived tree height measurements for two of the most ecologically and commercially significant species in western North America, Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa). Tree height measurements obtained from narrow-beam (0.33 m), high-density (6 points/m(2)) lidar were more accurate (mean error +/- SD = -0.73 +/- 0.43 m) than those obtained from wide-beam (0.8 m) lidar (-1.12 +/- 0.56 m). Lidar-derived height measurements were more accurate for ponderosa pine (-0.43 +/- 0.13 m) than for Douglas-fir (-1.05 +/- 0.41 m) at the narrow beam setting. Although tree heights acquired using conventional field techniques (-0.27 +/- 0.27 m) were more accurate than those obtained using lidar (-0.73 +/- 0.43 m for narrow beam setting), this difference will likely be offset by the wider coverage and cost efficiencies afforded by lidar-based forest survey.