Laser altimetry waveform measurement of vegetation canopy structure

A profiling airborne laser altimeter system operating at 1.06 μm has been developed to provide rapid, direct measurement of vegetation canopy structure by digitization of the amplitude versus time history of return laser pulse energy. The resulting waveforms are a measure of the vertical distribution of reflecting elements within a single laser footprint, including foliage, branches and ground. Vegetation height can be extracted from the waveform data based on the time difference between first and last returns. The altimeter system was deployed to the Pacific Northwest in September, 1993 where 5 flight missions were conducted over a variety of vegetated terrains, yielding approximately 4,000 km of georeferenced waveform profiles. Analysis of data from the Gifford Pinchot National Forest (GPNF) demonstrate good agreement between waveform-derived and ground-based measurements of vegetation height. The GPNF is a region of extensive timber harvesting, providing homogeneous areas of clear-cut, replanted conifers of increasing maturity, and old growth stands. Ground observations of canopy height and closure are available for 14 calibration stands that were differentiated, based on Landsat Thematic Mapper spectral characteristics, into 7 classes. Laser altimeter waveform data with a vertical sampling resolution of 22 cm were collected over calibration stands for 5 of the spectral classes. Last return pulses consistent with a ground measurement were acquired on every laser shot. Tree heights recovered from the altimeter waveforms show sub-meter agreement with ground based measurements, with essentially no vegetation cover on the clear cuts increasing to a mean tree height of 13 m on the most mature replanted sites. Altimeter-based tree heights for the old-growth stands typically range between 30 and 45 m, consistent with the ground measurements.