LiDAR

A LiDAR instrument emits a short pulse of laser light and records the time taken for reflected light to return. Since the pulse travels to a surface and back, the distance is calculated from half its round-trip travel time multiplied by the speed of light. A scanner directs pulses across the scene; a positioning system and an inertial measurement unit record where the instrument is and how it is oriented. Combining those measurements locates each return in three-dimensional space. A collection of such points is called a point cloud. Because the instrument supplies its own light, LiDAR is an active remote-sensing method, unlike passive imaging that depends on reflected sunlight or other external illumination.

LiDAR systems can be carried by aircraft, satellites, vehicles, handheld devices or stationary ground instruments. Airborne laser scanning covers broad areas, while terrestrial systems can measure detailed structures at closer range. Topographic LiDAR commonly uses near-infrared light to map land elevation. Bathymetric LiDAR uses green light that can travel through water to measure elevations of riverbeds and the seafloor in suitable survey conditions. Atmospheric LiDAR, used in research systems such as those at NASA's Jet Propulsion Laboratory, analyzes light scattered back from molecules and particles to study properties such as aerosols, ozone, water vapour, temperature and density.

A point cloud can be processed into elevation models, contours and three-dimensional representations of natural or built environments. Mapping agencies use LiDAR to improve terrain and shoreline information and to support emergency response. In forests, returns from branches, tree crowns and the ground can help researchers estimate canopy structure, tree heights and vegetation patterns. LiDAR has also helped archaeologists identify built features hidden beneath dense vegetation, and bathymetric systems can measure shallow-water terrain. These products are derived from measured returns: analysts classify and process the point cloud to separate features such as ground, buildings and vegetation for a particular task.

Attempts to use light to measure distance began before modern laser systems: searchlights were used to investigate the atmosphere in the 1930s, and pulsed light was used to estimate cloud heights in 1938. The invention of the laser in 1960 enabled a new generation of systems. Airborne LiDAR followed, but dependable georeferenced mapping became practical after GPS receivers and inertial measurement units became commercially available in the late 1980s. A point cloud is a dense set of measurements, not by itself a finished map: its interpretation depends on the sensor, survey conditions, calibration and the processing used to distinguish different surfaces. Multiple returns from one pulse, for example, can record reflections from tree crowns, branches and the ground.