From Bureau of Economic Geology, The University of Texas at Austin (
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AGU Fall Meeting, San Francisco, California, December 5–9, 2005

Comparison of GLAS and Small-Footprint,
Airborne Topographic Lidar Waveform Data from
Non-homogeneous Landcover in Central Texas

R. Gutierrez, A. L. Neuenschwander, M. M. Crawford, B. E. Schutz, T. Urban, and J. Liadsky


The Geoscience Laser Altimeter System (GLAS) on-board the Ice, Cloud and land Elevation Satellite (ICESat) is capable of producing range measurements with a precision of approximately 3 cm. While designed for mapping elevations of the ice sheets, data from GLAS have increasingly been used for vegetated terrestrial applications. Two ICESat ascending and descending passes were targeted over the Freeman Ranch, located near San Marcos, Texas on 14 March 2005 and 25 May 2005 to investigate the use of waveform lidar for determining vegetation structure. The Freeman Ranch is characterized by a mixture of rangeland and Oak-Juniper woodlands, and is the focus of many terrestrial studies focused on ecosystem carbon storage. The GLAS footprint during these acquisitions is approximately 35 m in diameter, which is a limitation for areas with non-homogeneous landcover such as the Freeman Ranch. To validate the potential of GLAS waveforms for determining vegetation structure, small-footprint airborne lidar data were collected by the Bureau of Economic Geology (BEG) on 12 August 2005 using an Optech ALTM 1225 system. The BEG ALTM system is equipped with a waveform digitizer that allows the simultaneous recording of conventional first and last return lidar data and the corresponding reflection waveforms at the 25 kHz laser pulse repetition rate. The small-footprint waveforms can be up-sampled to replicate the footprint of the GLAS system and scale differences between the two systems can be characterized and ultimately modeled. This project is intended to be a preliminary step towards developing a basis for relating airborne waveform lidar to spaceborne waveform lidar.