Airborne light detection and ranging (LIDAR) technology is an active remote sensing technology which allows accurate measurements of topography, vegetation canopy heights, and buildings over large areas. Most modern ALTM systems consist of three basic components: the laser scanner, a kinematic Global Position System (GPS), and the Inertial Measurement Unit (IMU). The laser scanner detects the range from aircraft to ground by recording the time difference between laser pulses sent out and reflected back. Many systems allow the recording of multiple returns and the return intensity for each laser pulse. Pulse repetition rates of commercial LIDAR systems range between 5 and 167 kHz. A rotating or oscillating mirror mounted in front of the laser causes the laser to scan back and forth, allowing the coverage of a wide swath beneath the flight path. This oscillation of the scanner mirror, in combination with forward motion of the aircraft, typically results in a zigzag scan pattern beneath the flight path.
A GPS receiver mounted in the aircraft records aircraft positions continuously. A second GPS station situated at a known ground position provides differential corrections for more accurate estimation of the aircraft trajectory. The IMU consists of a set of gyroscopes and accelerometers that continuously measure the roll, pitch, heading and acceleration of the aircraft. After the flight, the aircraft trajectory is then combined with the laser range data, scanner mirror angle, and the IMU measurements to determine the precise horizontal coordinates and vertical elevations of each laser reflection.
LIDAR at IHRC
The Florida International University (FIU) International Hurricane Research Center (IHRC) and the University of Florida (UF) Geomatics program purchased an Optech model 1210 ALTM system in 1999 and updated the system to 1233 later. The system is mounted in a Cessna 337 twin-engine light aircraft owned jointly by FIU and UF.
The Optech 1233 ALTM utilizes a 33 kHz, pulsed laser range finder (LIDAR) which returns vertical ranges to the ground on a swath beneath the flight path. When combined with advanced inertial navigation and kinematic GPS positioning, this system can return absolute elevations of the ground surface accurate to 15 cm (6 inches). For a typical aircraft deployment of 190 km (120 miles) per hour ground speed, 1200 m (3600 foot) altitude (Figure 1), we are able to map a 650-m-wide (2000-foot-wide), over 800-km-long (500-mile-long) swath of ground surface elevations spaced 1.5 m (4.5 foot) apart in just a few hours and at a fraction of the cost of conventional surveying.
Figure 1. Schematic diagram for LIDAR data acquisition
Since acquiring an airborne laser in 1999, IHRC has mapped about 2,650 km2 of area vulnerable to storm surge flooding in Broward, Palm Beach, Manatee, and Miami-Dade Counties using airborne LIDAR technology as part of the Windstorm Simulation and Modeling Project funded by FEMA through the Florida Department of Community Affairs (FL DCA). Over 3.2 billion irregularly spaced ground surface elevations have been collected for these areas. IHRC also collected LIDAR measurements for storm surge vulneravle areas in Martin County during 2003. In addition to mapping areas vulnerable to storm surge inundation, IHRC mapped Vero Beach, Florida (2000), the outer coastline of North and South Carolina (2000), the south shore of Long Island, New York (2002), Captiva Island, Florida (2004) to study coastal erosion induced by hurricanes. IHRC also mapped mash, pine fprests, and mangrove forests in Everglades National Park (2004, 2006, 2007) and Big Pine Key (2007) for vegetation classification and forest gap detections. These data collection activities are part of an extensive research effort that has contributed to the development of unique capabilities in LIDAR data filtering, building and tree extraction algorithms, data processing software development, storm surge and freshwater flood modeling, analysis of sea level rise impact, and coastal erosion using LIDAR measurements.