Assessment on the influence of flying height and scan angle on biophysical vegetation products derived from airborne laser scanning

摘要

Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical vegetation properties such as tree height, crown width, fractional cover and leaf area index (LAI). It is expected that the conditions of data acquisition, such as viewing geometry and sensor configuration influence the value of these parameters. In order to gain knowledge about these different conditions, we test for the sensitivity of vegetation products for viewing geometry, namely flying altitude and scanning (incidence) angle. Based on two methodologies for single tree extraction and derivation of fractional cover and LAI previously developed and published by our group, we evaluate how these variables change with either flying altitude and scanning angle. These are the two parameters which often need to be optimised towards the best compromise between point density and area covered with a single flight line. Our testsite in the Swiss National Park was overflown with two nominal flying altitudes, 500 and 900 m above ground. Incidence angle and local incidence angle were computed based on the digital terrain model using a simple backward geocoding procedure. We divided the raw laser returns into several different incident angle classes based on the flight path data; the TopoSys Falcon II system used in this study has a maximum scan angle of ±7.15o. We compare the derived biophysical properties from each of these classes with field measurements based on tachymeter measurements and hemispherical photographs, which were geolocated using differential GPS. It was found that with increasing flying height the well-known underestimation of tree height increases. A similar behavior can be observed for fractional cover; its respective values decrease with higher flying height. The behavior for incidence angles is not so evident, probably due to the small scanning angle of the system used. LAI seems to be most affected by incidence angles, with higher values for locations further away from nadir. Incidence angle seems to be of higher importance for vegetation density parameters than local incidence angle. We conclude that a more detailed knowlegde of beam-canopy interaction is needed, be it through empirical test such as ours or through using numerical models such as ray tracers.