Light Detection and Ranging or Laser Imaging Detection and Ranging (LiDAR) technology is a revolutionary tool in both cartography and forestry. This technology allows the distance from a laser emitter to an object or surface to be determined using a pulsed laser beam. The distance to the object is determined by measuring the time delay between the emission of the pulse and its detection through the reflected signal.
It is usually used for the acquisition of terrain points in the form of a three-dimensional point cloud, mainly used to collect accurate and productive elevation data in topographic applications, especially because it allows the capture of a massive point cloud on different types of land surfaces (vegetation, buildings, soil…). But it also has a multitude of applications in fields such as geology, seismology and atmospheric physics.
Among the most innovative applications of LiDAR technology is the autonomous driving of transport vehicles where the technological challenge is the acquisition and processing of the point cloud in real time.
On the other hand, we can find satellite LiDAR, where the sensor is located on satellites covering large areas with less detail.
LiDAR components and specifications
Airborne LiDAR consists of a laser sensor installed in an aircraft together with a Global Positioning System (GPS) which provides absolute positioning for the LiDAR technology and an Inertial Navigation System (INS) which determines the orientation. As a result, precise measurements of the terrain are obtained in the form of a three-dimensional point cloud. In addition to this type of LiDAR, there are also terrestrial LiDARs that can be installed in a vehicle or mounted on a tripod and which are currently being applied to develop autonomous driving for transport vehicles.
Main specifications of a LiDAR system
- Pulse frequency: number of pulses per second. The higher the frequency, the more points obtained. Nowadays, frequencies higher than 150 kHz are used.
- Flight altitude and speed: the higher the altitude and the faster the speed, the lower the cost of the flight. However, when the altitude decreases, the plane flies closer to the ground and precision increases.
- Scanning pattern: path taken by the laser beam. The four main patterns are: linear, zigzag, elliptical and fibre optic.
- Scanning frequency: number of scan lines per second.
- Beam divergence: deviation of the photons from the beam line, the greater the distance the greater the diameter of the beam.
- Scanning angle: angle of the pulse perpendicular to the line of flight, which determines the Field of View (FOV). The smaller the FOV, the more detailed the scan.
- Footprint diameter: sampling area occupied by the beam in a plane, which differs according to the flight height and beam divergence.
- Distance between tracks: distance between the beams in the line of flight. Distance between the surfaces covered by two consecutive beams.
- Pulse length: duration of the pulse emission in nanoseconds.
- Wavelength: the type of sensor with its corresponding wavelength is determined according to the type of surface to be measured. To measure the terrain, work is done in the near infrared (1040-1060nm), while for bathymetry, in addition to the infrared, work is done in the green region (500-600nm).
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