How did we get here? An early history of forestry lidar

Functioning lasers were first demonstrated in 1960 in the United States and in 1961 in Canada and in the Soviet Union, but research into the use of lasers as forest measurement tools did not begin for another 15 years. Initially, with respect to Earth resources, lasers were employed to measure sea ice surface roughness, to make near-shore bathymetric measurements, to penetrate forests to make detailed topographic measurements, and to fluoresce oceanic phytoplankton for surface current studies. Some of these early studies noted that forest profiles were evident but in fact added noise to topographic retrievals. As early as 1964, researchers noted vegetation returns acquired using an airborne helium-neon (He-Ne), 0.63 mm, continuous wave (CW) laser. A decade and many airborne studies later, scientists with TRANARG, a mapping and surveying company in Caracas, Venezuela, reported on flights undertaken in 1976 that utilized a He-Ne lidar to collect over 11 000 km of lidar profiles, spaced 1.5 km apart, to construct a topographic map to help site a new reservoir. Though they depended on the laser to penetrate vegetation, they noted 35-40 m median canopy heights with emergents up to 55 m in their profiles. Trees came to be regarded as a signal rather than noise in the mid-1970s. In 1976 in the Soviet Union, Russian researchers felled a birch and a spruce, aimed a He-Ne CW laser with a spot size of approximately 25 mm at the horizontal trees, produced a profilograph, compared it with tape measurements, and concluded that, with increased power, such a laser could be mounted on an aircraft to remotely measure forest canopies. In 1979, they mounted their He-Ne laser on an AN-2 biplane and acquired their first airborne profiles. These studies and others done prior to 1985, i.e., the first two decades of airborne laser research, are reviewed in this glance backwards at the history of forestry lidar.