1. Full-waveform, Laser Altimeter Measurements of Vegetation Vertical Structure and Sub-Canopy Topography in Support of the North American Carbon Program
    Blair, B.; Hofton, M. R. D. P. P. R. J.  AGU  December 2004

    Full-waveform, scanning laser altimeters (i.e. lidar) provide a unique and precise view of the vertical and horizontal structure of vegetation across wide swaths. These unique laser altimeters systems are able to simultaneously image sub-canopy topography and the vertical structure of any overlying vegetation. These data reveal the true 3-D distribution of vegetation in leaf-on conditions enabling important biophysical parameters such as canopy height and aboveground biomass to be estimated with unprecedented accuracy. An airborne lidar mission was conducted in the summer of 2003 in support of preliminary studies for the North America Carbon Program. NASA's Laser Vegetation Imaging Sensor (LVIS) was used to image approximately 2,000 km2 in Maine, New Hampshire, Massachusetts and Maryland. Areas with available ground and other data were included (e.g., experimental forests, FLUXNET sites) in order to facilitate numerous bio- and geophysical investigations. Data collected included ground elevation and canopy height measurements for each laser footprint, as well as the vertical distribution of intercepted surfaces (i.e. the return waveform). Data are currently available at the LVIS website (http://lvis.gsfc.nasa.gov/). Further details of the mission, including the lidar system technology, the locations of the mapped areas, and examples of the numerous data products that can be derived from the return waveform data products are available on the website and will be presented. Future applications including potential fusion with other remote sensing data sets and a spaceborne implementation of wide-swath, full-waveform imaging lidar will also be discussed.



  2. Assessing the Ability of Laser Altimeter Return Waveforms To Detect Surface Topographic Change
    Hofton, M. A.; Blair, J. B. R. D. L. P. P.  AGU  December 2004

    The capability of medium/large footprint (10 m or greater) full-waveform laser altimeters to penetrate beneath dense vegetation to directly measure sub-canopy topography provides a unique capability for sensing topographic change in the presence of vegetation. In this study, we assess the ability of geolocated laser altimeter return waveforms instead of individual elevation measurements to measure vertical elevation change. The method we use (dubbed the return pulse correlation method) maximizes the shape similarity of near-coincident, vertically-geolocated laser return waveforms from two observation epochs as they are vertically-shifted relative to each other. Using waveform data collected by NASA's Laser Vegetation Imaging Sensor (LVIS) we assess the inherent precision of elevation change measurements derived using the return pulse correlation method. Data collected in 1999, 2003 and 2004 are compared using the pulse correlation method and by simple differencing of coincident elevation measurements. Data collected under "bare earth" conditions as well as in the presence of dense, growing vegetation are utilized, demonstrating the usefulness of this technique for complementing and extending earth surface dynamics measurements made using other remote sensing techniques