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W. Wagner, M. Hollaus, C. Briese, V. Ducic:
"3D vegetation mapping using small-footprint full-waveform airborne laser scanners";
International Journal of Remote Sensing, 29 (2008), 5; 1433 - 1452.

Small-footprint full-waveform airborne laser scanning (ALS) is a remote sensing
technique capable of mapping vegetation in three dimensions with a spatial
sampling of about 0.5-2m in all directions. This is achieved by scanning the laser
beam across the Earth´s surface and by emitting nanosecond-long infrared pulses
with a high frequency of typically 50-150 kHz. The echo signals are digitized
during data acquisition for subsequent off-line waveform analysis. In addition to
delivering the three-dimensional (3D) coordinates of scattering objects such as
leaves or branches, full-waveform laser scanners can be calibrated for measuring
the scattering properties of vegetation and terrain surfaces in a quantitative way.
As a result, a number of physical observables are obtained, such as the width of
the echo pulse and the backscatter cross-section, which is a measure of the
electromagnetic energy intercepted and re-radiated by objects. The main aim of
this study was to build up an understanding of the scattering characteristics of
vegetation and the underlying terrain. It was found that vegetation typically
causes a broadening of the backscattered pulse, while the backscatter crosssection
is usually smaller for canopy echoes than for terrain echoes. These
scattering properties allowed classification of the 3D point cloud into vegetation
and non-vegetation echoes with an overall accuracy of 89.9% for a dense natural
forest and 93.7% for a baroque garden area. In addition, by removing the
vegetation echoes before the filtering process, the quality of the digital terrain
model could be improved.

http://dx.doi.org/10.1080/01431160701736398