Abstract
Abstract. Full-waveform is becoming increasingly available in today's LiDAR systems and the analysis of the full return signal can provide additional information on the reflecting surfaces. In this paper we present the results of an assessment on full-waveform analysis, as opposed to the more classic discrete return analysis, for discerning vegetation cover classes related to post-fire renovation. In the spring of 2011 an OPTECH ALTM sensor was used to survey an Alpine area of almost 20 km2 in the north of Italy. A forest fire event several years ago burned large patches of vegetation for a total of about 1.5 km2 . The renovation process in the area is varied because of the different interventions ranging from no intervention to the application of re-forestation techniques to accelerate the process of re-establishing protection forest. The LiDAR data was used to divide the study site into areas with different conditions in terms of re-establishment of the natural vegetation condition. The LiDAR survey provided both the full-waveform data in Optech's CSD+DGT (corrected sensor data) and NDF+IDX (digitizer data with index file) formats, and the discrete return in the LAS format. The method applied to the full-waveform uses canopy volume profiles obtained by modelling, whereas the method applied to discrete return uses point geometry and density indexes. The results of these two methods are assessed by ground truth obtained from sampling and comparison shows that the added information from the full-waveform does give a significant better discrimination of the vegetation cover classes.
Highlights
Airborne LiDAR in the past ten years has seen rapid growth in both sensor technology and fields of application
Research on laser profilers started in the late nineties and a lot of interest was given to the forestry environment because of the ability of the laser pulse to penetrate canopy, returning ground hits, which are precious for digital terrain models (DTMs) extraction (Carson et al, 2004)
If one laser pulse gets reflected by different surfaces along its path, interactions of the laser with the objects cause a return signal whose characteristics are a mixture derived from the different optical properties of the objects, the range and the incidence angle (Wagner et al, 2006)
Summary
Airborne LiDAR (or Airborne Laser Scanning - ALS) in the past ten years has seen rapid growth in both sensor technology and fields of application. Studies over the rate of penetration show that typical coniferous and deciduous forests allow 20-40% of the laser pulses to return ground hits in leaf-on conditions and as much as 70% in leaf-off conditions (Ackermann, 1999). This can occur because the size of the diffraction cone (Mallet and Bretar, 2009) can vary from a few centimeters up to one/two meters; the canopy area that is illuminated is large enough to have a significant amount of gaps which allow part of the laser energy to pass without getting reflected by leaves of branches, all the way to the ground, which is the element which causes the last reflection. If one laser pulse gets reflected by different surfaces along its path, interactions of the laser with the objects cause a return signal whose characteristics are a mixture derived from the different optical properties of the objects, the range and the incidence angle (Wagner et al, 2006)
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