Progress Towards Global Estimates of Forest Canopy Height from Geoscience Laser Altimeter System Waveforms

The vertical extent of waveforms collected by the Geoscience Laser Altimeter System (onboard ICESat - the Ice, Cloud, and land Elevation Satellite) increases as a function of terrain slope and footprint size (the area on the ground that is illuminated by the laser). Over sloped terrain, returns from both canopy and ground surfaces can occur at the same elevation. As a result, the height of the waveform (waveform extent) is insufficient to make estimates of tree height on sloped terrain, and algorithms are needed that are capable of retrieving information about terrain slope from the waveform itself. Early work on this problem used a combination of waveform height indices and slope indices from a digital elevation model (DEM). A second generation algorithm was developed using datasets from diverse forests in which forest canopy height has been estimated in the field or by via airborne lidar. Forest types originally considered in the development of the second generation algorithm included evergreen needleleaf, deciduous broadleaf and mixed stands in temperate North America, and tropical evergreen broadleaf forests in Brazil. Drawing on the work of our collaborators, we now include data from North America, European and Asian boreal forests, temperate needleleaf forests in Asia and North America, and tropical broadleaf forests in Central America, Africa and Southeast Asia. The resulting equations are the first step in creating global estimates of forest canopy height and aboveground biomass.