Lidar can be defined as the integration of three technologies into a single system capable of acquiring data to produce accurate digital elevation models (DEMs). Discussion over Lidar and photogrammetry. These technologies are lasers, the Global Positioning System (GPS), and inertial navigation systems (INS). A complete Lidar manual available for free download. Combined, they allow the positioning of the footprint of a laser beam as it hits an object, to a high degree of accuracy. Lasers themselves are very accurate in their ranging capabilities, and can provide distances accurate to a few centimeters. The accuracy limitations of LiDAR systems are due primarily to the GPS and IMU (Inertial Measurement Unit) components. As advancements in commercially available GPS and IMUs occur, it is becoming possible to obtain a high degree of accuracy using LiDAR from moving platforms such as aircraft.
A LiDAR system combines a single narrow-beam laser with a receiver system. The laser produces an optical pulse that is transmitted, reflected off an object, and returned to the receiver. The receiver accurately measures the travel time of the pulse from its start to its return. With the pulse travelling at the speed of light, the receiver senses the return pulse before the next pulse is sent out. Since the speed of light is known, the travel time can be converted to a range measurement. Combining the laser range, laser scan angle, laser position from GPS, and laser orientation from INS, accurate x, y, z ground coordinates can be calculated for each laser pulse. Laser emission rates can be anywhere from a few pulses per second to tens of thousands of pulses per second. Thus, large volumes of points are collected. For example, a laser emitting pulses at 10,000 times per second will record 600,000 points every minute. Typical raw laser point spacing on the ground ranges from 2 to 4 meters.
Some LiDAR systems can record “multiple returns” from the same pulse. In such systems the beam may hit leaves at the top of tree canopy, while part of the beam travels further and may hit more leaves or branches. Some of the beam is then likely to hit the ground and be reflected back, ending up with a set of recorded “multiple returns” each having an x, y, z position. This feature can be advantageous when the application calls for elevations for not only the ground, but for tree or building heights. As surface types and characteristics vary and change the laser beam’s reflectivity, then the ability of the LiDAR to record the return signals changes. For example, a laser used for topographic applications will not penetrate water, and in fact records very little data even for the surface of the body of water. Where the application calls for a laser to penetrate water to determine x, y, z positions of undersea features, then a slightly different variation of LiDAR technology is used.
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