Ray tracing for modeling of small footprint airborne laser scanning returns

Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical canopy variables, such as treeheight and vegetation density. However, up to now most approaches are built upon empirical stand based methods for linking ALS datawith the relevant canopy properties estimated by field work. These empirical methods mostly comprise regression models, where effectsof site conditions and sensor configurations are contained in the models. Thus, these models are only valid for a specific study, whichrenders inter-comparison of different approaches difficult. Physically based approaches exist e.g. for the estimation of tree height andtree location, however systematic underestimation depending upon sampling and vegetation type remains an issue. Using a radiativetransfer model that builds on the foundation of the Open-Source ray tracer povray we are simulating return signals for two ALS systemsettings (footprint size and laser wavelength). The tree crowns are represented by fractal models (L-systems), which explicitly resolvethe position and orientation of single leafs. The model is validated using ALS data from an experiment with geometric reference targets.We were able to reproduce the effects of target size and target reflectance that were found in the real data with our modeling approach.A sensitivity study was carried out in order to determine the effect of properties such as beam divergence (0.5, 1, and 2 mrad), canopyreflectance (laser wavelength, 1064 and 1560 nm) on the ALS return statistics. Using the two laser wavelengths above, we were ableto show that the laser wavelength will not significantly influence discrete return statistics in our model. It was found that first echoreturn statistics only differ significantly if the footprint size was altered by a factor of 4. Last return distributions were significantlydifferent for all three modelled footprint sizes, and we were able to reproduce the effect of an increased number of ground returns forlarge footprint sizes. These forward simulations are a first step in the direction of physically based derivation of biophysical ALS dataproducts and could improve the accuracy of the derived parameters by establishing correction terms.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, XXXVI (3/W52)
ISSN:1682-1750
ISSN:2194-9034
ISSN:1682-1777