Abstract
Abstract. The characterization of the vertical forest structure is highly relevant for ecological research and for better understanding forest ecosystems. Full-waveform airborne laser scanner systems providing a complete time-resolved digitization of every laser pulse echo may deliver very valuable information on the biophysical structure in forest stands. To exploit the great potential offered by full-waveform airborne laser scanning data, the development of suitable voxel based data analysis methods is straightforward. Beyond extracting additional 3D points, it is very promising to derive voxel attributes from the digitized waveform directly. However, the ’history’ of each laser pulse echo is characterized by attenuation effects caused by reflections in higher regions of the crown. As a result, the received waveform signals within the canopy have a lower amplitude than it would be observed for an identical structure without the previous canopy structure interactions (Romanczyk et al., 2012). 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 by applying suitable attenuation correction models. The basic idea of the correction procedure is to enhance the waveform intensity values in lower parts of the canopy for portions of the pulse intensity, which have been reflected in higher parts of the canopy. To estimate the enhancement factor an appropriate reference value has to be derived from the data itself. Based on pulse history correction schemes presented in previous publications, the paper will discuss several approaches for reference value estimation. Furthermore, the results of experiments with two different data sets (leaf-on/leaf-off) are presented.
Highlights
The scientific investigation of forest ecosystems with respect to forest-related research like carbon pools and greenhouse gas emissions (Jandl et al, 2015), silvicultural questions like lower tree crown delineation and the density of understory vegetation (Spies, 1998), as well as meteorological applications like flow simulation in forest stands (Queck et al, 2012) and gas exchange (Foken et al, 2012) require a high-resolution characterization of the vertical forest structure
The differential backscatter cross section is estimated by deconvolution techniques (Roncat et al, 2011) and projected into a Cartesian voxel structure
The differential backscatter cross section resulting from deconvolution is in first instance only an apparent cross section, because the measuring process is influenced by different attenuation effects
Summary
The scientific investigation of forest ecosystems with respect to forest-related research like carbon pools and greenhouse gas emissions (Jandl et al, 2015), silvicultural questions like lower tree crown delineation and the density of understory vegetation (Spies, 1998), as well as meteorological applications like flow simulation in forest stands (Queck et al, 2012) and gas exchange (Foken et al, 2012) require a high-resolution characterization of the vertical forest structure. Small-footprint full-waveform laser scanning systems enable the measurement of physical and geometric attributes of the vegetation canopy structure. In contrast to existing approaches based on the extraction of discrete 3D-points via a gaussian decomposition, it is very promising to derive the voxel attributes from the digitized waveform directly. For this purpose, the differential backscatter cross section is estimated by deconvolution techniques (Roncat et al, 2011) and projected into a Cartesian voxel structure. The geometric aspects include an intersection of the diverging laser pulse cone with the voxel structure and an interpolation procedure to obtain the actual voxel values. The amplitudes of the backscatter cross section have to undergo some radiomet-
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