A Portable Airborne Laser System for Forest Inventory

were first applied to terrestrial concerns throughout the 1970s Abstract (Link and Collins, 1981; Hoge et al., 1983; Krabill et al., 1984). A simple, lightweight, inexpensive, portable airborne laser Over the past 30 years, the technology hasmoved from research profiling system has been assembled from off-the-shelf, comto commercial applications (Baltsavias, 1999). Applications mercially available components. The system, which costs include the use of airborne laser data for creation of digital terapproximately $30,000, is designed to fly aboard small rainmodels, monitoring power lines, urban areamapping, and helicopters and singleor twin-engine high-wing aircraft land-cover surveys (e.g., http://www.airbornelasermapping. without airframe modification. The system acquires firstcom, last accessed 26 August 2002). Numerous commercial return range and amplitude measurements at data rates up companies nowoffer turnkey airborne laser scanning systems or to 2000 hz (operator-controlled) and has an operational enfor-hire airborne laser data collection. velope up to 300 m above terrain. The airborne laser profiling Applications of laser technology to forest inventory probsystem includes the laser transmitter/receiver, differential GPS lems, however, remain largely in the research arena, for the folreceiver, a CCD video camera and recorder, and a laptop comlowing reasons: puter which interleaves and records the GPS and laser range/ amplitude data. The portable airborne laser system (PALS) was ● Laser data post-processing to derive inventory estimates requires the attention of specialists and, frequently, the applicadesigned to acquire forest height measurements along linear tion of specialized, home-grown software; flight transects in order to conduct regional or subcontinental ● Although forest/laser research has been ongoing for at least two forest inventories worldwide. This economical laser system decades, it is only in the last few years that researchers have now puts airborne laser mensuration within reach of operademonstrated beyond a reasonable doubt that laser data can be tional foresters and researchers interested in making rapid used to create reliable maps of tree height, canopy density, bioforest structure and/or timber surveys in remote areas. PALS mass, and volume; and has been used to acquire over 5000 km of flight transect data ● The flight hardware, to this point, has been expensive. over the state of Delaware. Certainly, some of the more sophisticated commercial and research laser scannersmaintain price tags in the million dollar Introduction plus range. Research systems such as SLICER (Scanning Lidar Airborne lasers may be used to acquire ranging data to measure Imager of Canopies by Echo Recovery; Blair et al., 1994; Hartree heights. The tree height datamay be used directly tomeasding et al., 1994), LVIS (Laser Vegetation Imaging Sensor; Blair ure such forest biophysical characteristics as average canopy et al., 1999), AOL (Airborne Oceanographic Lidar; Krabill et al., height, height variability or canopy roughness, and canopy 1984), and EAARL (Experimental Advanced Airborne Research closure (Arp et al., 1982; Nelson et al., 1984; Aldred and Lidar) are experimental, one-of-a-kind systems that are deBonner, 1985; Schreier et al., 1985; Ritchie et al., 1992; Ritchie signed to push technological envelopes rather than to be costet al., 1993; Nilsson, 1996; Næsset, 1997a; Blair et al., 1999; effective. Commercial airborne lasers currently sold are Means et al., 1999; Means et al., 2000; Popescu et al., 2002). sophisticated, turn-key scanning systems used primarily to creThese height and density measurements, in turn, can serve as ate accurate airborne digital elevation models (DEMs). These the independent variables in predictivemodels to estimate forlaser scanning systems record precise range and positioning est basal area, merchantable volume, biomass, and carbon data so that georeferenced DEMs can be produced. Some com(Maclean and Krabill, 1986; Nelson et al., 1988a; Nelson et al., mercially available, airborne laser scanning systems host 1988b; Nelson et al., 1997; Næsset, 1997a; Næsset, 1997b; Lefmulti-return receivers. Data from these multiple-return, laser sky et al., 1999a; Lefsky et al., 1999b; Popescu et al., 2000). scanning systems can be used to make DEMs of the top of the A wide variety of laser systems now exist, and numerous forest canopy, the ground, and significant subcanopy layers. researchers are investigating uses of these systemswith respect Commensurate with this level of sophistication are price tags of to forestry applications. Originally designed for bathymetry hundreds of thousands of dollars, which effectively puts these (Hickman and Hogg, 1969; Hoge et al., 1980), airborne lasers data out of the reach of operational foresters. The authors, being interested in the uses of laser data for large-area forest inventory and forest canopy characterization, set about to design a simple, inexpensive, portable airborne R. Nelson is with the Biospheric Sciences Branch, Code 923, laser system. Their objective was to design, build, test, and NASA/Goddard Space Flight Center, Greenbelt, MD 20771 ([email protected]).