Comparison of laser and field measurements of vegetation height and canopy cover

Distribution of vegetation properties is fundamental for understanding vegetation patterns and characteristics, improving estimates of infiltration, evapotranspiration, and soil erosion. A laser altimeter mounted in a small airplane was used to measure surface patterns of the landscape on the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed near Tombstone, Arizona. The airborne laser altimeter is a pulsed gallium-arsenide diode laser, transmitting and receiving 4000 pulses per second at a wavelength of 0.904 μm. The laser has a 1-mrad field of view and is designed to have a vertical recording precision of 0.05 m on a single measurement. Aircraft altitude varied between 100 and 300 m for the flights. Digital data from the laser were collected with a portable computer and analyzed to provide information on changes in vegetation height, spatial patterns, and patchiness of vegetation cover. The laser-measured vegetation properties of plant height and canopy cover (>0.3 m) were not significantly different than field measurements made using the line-intercept transect method at seven of the eight sites evaluated. Although the laser measurements of canopy height were not significantly different from the ground measurements, the laser consistently overestimated canopy cover less than 0.3 m in height and underestimated canopy cover greater than 0.5 m. New techniques to discriminate the background noise in the laser return signal in sparsely populated shrub communities are necessary before this technique will be fully useful in estimating canopy cover on rangelands. These studies indicate the potential of airborne laser to measure vegetation patterns quickly and quantitatively. The laser also has the ability to separate and map distinctly different plant communities.