Assessing laser pulse penetration in spruce canopies - combining field measured branch properties with discrete return airborne laser scanning data

摘要

In forest inventory, airborne laser scanning (ALS) has long been used to estimate various biophysical properties, such as stand volume and mean tree heights. It is also possible to use ALS to detect and measure single trees allowing the derivation of even more detailed information. However, even if the laser measurements are accurate one specific challenge occurs with tree canopies - they are not solid objects, and the laser echoes are not necessarily returned from the surface of the canopy. Thus, naive algorithms might not reproduce the spatial extent of each tree crown correctly. This can result in biased estimates if crown attributes are used in subsequent statistical analyses. The aim for this study was to assess the laser penetration into tree crowns by comparing field measured branch structures with high density ALS data. The material consisted of measured branches from 15 trees, where branch diameter and branch base height had been measured. Precisely measured tree positions allowed the calculation of geographical coordinates for each measured branch tip. A crown surface assumed to be the true crown extent was constructed using a convex hull algorithm.. The ALS data were collected with a discrete return scanning system, Optech 3100EA, with known pulse level orientation parameters, such as aircraft position and mirror angle. Each pulse was intersected with the crown surface model and the distance from the intersection to the pulse return was calculated. This distance was then used to analyze patterns of the pulse penetration, given scan angle and angle of incidence when intersecting the canopy. The results revealed no strong influence of scan angle or angle of incidence on the penetration.