Lidar is a rapidly evolving surveying technology for collecting 3D point-clouds both from airborne platforms and in land-survey settings. This magazine regularly devotes in-depth features to the principles and applications of this important emergent technology and, with the inclusion of a Product Survey, Technology in Focus and no fewer than four articles on terrestrial Lidar, it is the main theme of this issue. Terrestrial Lidar makes possible the swift measurement of millions of points by automatically scanning the scene at high speed. In the resulting dense point-cloud objects can be easily identified, allowing the creation of 3D-models with a level of detail impossible to achieve within a reasonable space of time using traditional technologies. Terrestrial Lidar thus opens up new dimensions in surveying.
Laser scanners are active sensors that emit laser beams for measuring the distances to objects without human/object contact. The principle is based on either time-of-flight or phase-shift. In the time-of-flight technology the sensor emits a laser pulse in the direction of the object; the time taken by the part of the pulse reflected back to reach the instrument is measured. Distance is calculated by multiplying this travel time by the speed of light and dividing the result by two. In phase-shift technology the sensor emits laser beams which are modulated as sine waves. The phase of the reflected part of the laser beam is measured and compared to the phase of the outgoing one, and distance then calculated from the difference in phase (phase-shift). Point density is usually so high that by recording the strength of the reflected signal quasi images can be created.
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