Lidar (laser detection and ranging) is an optical Remote sensing technology which mainly USES the laser as the emitting light source and adopts the active Remote sensing equipment of photoelectric detection technology. Primarily used in airborne laser mapping applications, it is increasingly becoming a cost-effective alternative to traditional measurement techniques such as photogrammetry.
Lidar consists of transmitting system, receiving system and information processing. It USES lasers to intensively sample the earth's surface to produce high-precision measurements of x,y and z. It can generate discrete multi-point cloud data sets that can be managed, displayed, analyzed and Shared by ArcGIS.
After the completion of lidar data collection and measurement, the point data will be processed into highly accurate geographic registration x,y and z coordinates by analyzing the time range of laser, scanning Angle of laser, GPS position and INS information.
Laser pulses fired from lidar systems reflect off the surface of the earth and objects on it: vegetation, buildings, Bridges and so on. A laser pulse emitted may return to the lidar sensor in the form of one or more echoes. Any laser pulse emitted as it travels toward the ground will be split into as many echoes as the reflected surface if multiple surfaces are encountered.
The first laser pulse to return is the most important echo, which will be associated with the highest elements of the earth's surface, such as tree tops or building tops. The first echo may also represent the ground, in which case only one echo is detected by the lidar system.
Multiple echoes can detect the elevation of multiple objects within the laser feet of the outgoing laser pulse. The middle echo usually corresponds to the vegetation structure, while the final echo corresponds to the terrain model.
The final echo does not always return from the ground. Consider, for example, a case in which a pulse, on its way to the ground, hits a thick branch and does not reach the ground at all. In this case, the final echo returned not from the ground, but from the branch that reflected the entire laser pulse.
After processing, the spatially organized lidar data is called point cloud data. The initial point cloud is a large set of 3D elevation points, including x value, y value, z value, GPS timestamp and other attributes. After the initial lidar point cloud is postprocessed, the specific surface elements encountered by the laser can be classified. Ground, buildings, forest canopies, highways, and any objects encountered by the laser beam in the course of the measurements make up the laser-point cloud data.
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