Assessing effects of positioning errors and sample plot size on biophysical stand properties derived from airborne laser scanner data

Canopy height distributions were created from small-footprint airborne laser scanner data with an average sampling density of 1.1 points.m(-2) collected over 132 sample plots and 61 stands. Field measurements of each plot were carried out within two concentric circles (200 m(2) and 300 or 400 m(2)). The plot positions were altered randomly with Monte Carlo simulations. For various metrics derived from the canopy height distributions, the mean and the standard deviation (SD) of the differences between incorrect plot positions and ground-truth positions were compared. In general, SD was smaller for large field plots than for small plots, and the variation in SD among the Monte Carlo repetitions was smaller for large sample plots. The combined effects of field plot size and sample plot position error on the accuracy of mean tree height (h(L)), stand basal area (G), and stand volume (V) predicted at stand level using a two-stage procedure combining field training data and laser data were assessed. Standard deviation of the differences between predicted and observed h(L) was quite stable and of similar size for position errors up to 5 m. However, for G and V the influence of plot position error was more pronounced.