What is Lidar?
Light Detection and Ranging (LIDAR) is a technology similar to RADAR that can be used to create high-resolution digital elevation models (DEMs) with vertical accuracy as good as 10 cm. LIDAR equipment, which includes a laser scanner, a Global Positioning System (GPS), and an Inertial Navigation System (INS), is generally mounted on a small aircraft. The laser scanner transmits brief laser pulses to the ground surface, from which they are reflected or scattered back to the laser scanner. Detecting the returning pulses, the equipment records the time that it took for them to go from the laser scanner to the ground and back. The distance between the laser scanner and the ground is then calculated based on the speed of light. While flying, the airplane’s position is determined using GPS, and the direction of the laser pulses are determined using the INS. Because one laser pulse may reflect back from multiple surfaces, such as the top of a tree, a house, and the ground surface, there are multiple returns from each pulse that can be used to map such things as the top of the tree canopy, buildings, and the ground. Post-processing is used to differentiate between these multiple returns to determine the bare-earth surface. Using the combined information from the laser scanner, the GPS, and the INS, very accurate, closely spaced (typically 1 per square meter) X, Y, Z coordinates are determined from which a DEM is be made. -- Linda Mark USGS/Cascades Volcano Observatory, October 2004 Press release, October 25, 2004 -- [PDF format,32K] Preliminary color map of elevation change at Mount St. Helens, September 2003 to October 4, 2004. -- [PDF format,2.4M] Area shown is 5.2 km x 5.2 km in size, or about 3.25 miles square. Please note that small elevation differences, between -0.4 and +1.4 meters, are not shown as most such small differences reflect differing geodetic frameworks for the September 2003 and October 4, 2004 surveys. These decrease with further analysis and processing. Shaded relief images of Mount St. Helens showing recent volcanic unrest-related changes, including both still and animated versions. Synthetic Fly-throughs of Mount St. Helens crater, October 4, 2004 (slow). -- [MPEG format,3.9M] Created using IKONOS 1 meter panchromatic satellite image draped on October 4, 2004 topography, in "mpeg" format. IKONOS satellite image from Space Imaging. Fly-through courtesy NASA. Synthetic Fly-throughs of Mount St. Helens crater, October 4, 2004 (fast). --[MPEG format,475K] Created using IKONOS 1 meter panchromatic satellite image draped on October 4, 2004 topography, in "mpeg" format. IKONOS satellite image from Space Imaging. Fly-through courtesy NASA. Notes:
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