Attenuation correction of full-waveform airborne laser scanner data for improving the quality of volumetric forest reconstructions by simplified waveform history analysis

Full-waveform airborne laser scanning data yield a great amount of information for the analysis of vegetation density in forestry applications. So far, the processing of full-waveform laser scanner data in forestry applications has mostly been limited to the extraction of discrete maxima via a Gaussian decomposition. This will usually lead to a densification of the 3D point cloud between terrain model and crown model by 3-5 extra 3D points per laser pulse, plus information on echo width and amplitude. However, beyond this extraction of additional discrete 3D points, full waveform data may form a valuable basis for a fully volumetric representation of forest stands in a 3D voxel structure, wherein the voxel attributes are derived from the digitized waveform directly.
For this purpose, differential backscatter cross sections, derived from the digitized pulse echoes, have to be projected into a Cartesian voxel structure of a suitable resolution, wherein the voxel entries represent amplitudes of the cross section and can be interpreted as a local measure for the amount of pulse reflecting matter. However, the 'history' of each laser echo pulse is characterized by attenuation effects caused by reflections in higher regions of the crown. To achieve a radiometrically correct voxel space representation, the loss of signal strength caused by partial reflections on the path of a laser pulse through the canopy has to be compensated. Thereby, the correction term has to be derived from the digitized pulse echo itself. In this paper, we present an approach for a discrete correction, realized as a segment-based modification of the echo waveform amplitude. The basic idea of the procedure is to enhance the waveform intensity values in lower parts of the tree crown for portions of the pulse intensity, which have been reflected (and thus blocked) in higher parts of the crown. The paper will discuss the developed model of attenuation correction and show results from a validation both with synthetic and real world data.