What is LIDAR?

Lidar

Lidar is a method for determining ranges by targeting an object with a laser and measuring the time for the reflected light to return to the receiver. It can also be used to make digital 3-D representations of areas on the earth's surface and ocean bottom by varying the wavelength of light. It has terrestrial, airborne, and mobile applications. airborne, and mobile applications.

Lidar is an acronym of "light detection and ranging" or "laser imaging, detection, and ranging". It is sometimes called 3-D laser scanning, a special combination of 3-D scanning and laser scanning.

History

Under the direction of Malcolm Stitch, the Hughes Aircraft Company introduced the first lidar-like system in 1961, shortly after the invention of the laser. Intended for satellite tracking, this system combined laser-focused imaging with the ability to calculate distances by measuring the time for a signal to return using appropriate sensors and data acquisition electronics. It was originally called "Colidar" an acronym for "coherent light detecting and ranging," derived from the term "radar", itself an acronym for "radio detection and ranging".

What Is Lidar?

Lidar, which is commonly spelled LiDAR and also known as LADAR or laser altimetry, is an acronym for light detection and ranging. It refers to a remote sensing technology that emits intense, focused beams of light and measures the time it takes for the reflections to be detected by the sensor. This information is used to compute ranges, or distances, to objects. In this manner, lidar is analogous to radar (radio detecting and ranging), except that it is based on discrete pulses of laser light. The three-dimensional coordinates (e.g., x,y,z or latitude, longitude, and elevation) of the target objects are computed from

1) the time difference between the laser pulse being emitted and returned,

2) the angle at which the pulse was “fired,” and

3) the absolute location of the sensor on or above the surface of the Earth.

Figure 1 Lidar point and surface products

There are two classes of remote sensing technologies that are differentiated by the source of energy used to detect a target: passive systems and active systems. Passive systems detect radiation that is generated by an external source of energy, such as the sun, while active systems generate and direct energy toward a target and subsequently detect the radiation. Lidar systems are active systems because they emit pulses of light (i.e. the laser beams) and detect the reflected light. This characteristic allows lidar data to be collected at night when the air is usually clearer and the sky contains less air traffic than in the daytime. In fact, most lidar data are collected at night. Unlike radar, lidar cannot penetrate clouds, rain, or dense haze and must be flown during fair weather.

Lidar instruments can rapidly measure the Earth’s surface, at sampling rates greater than 150 kilohertz (i.e., 150,000 pulses per second). The resulting product is a densely spaced network of highly accurate georeferenced elevation points (Figure 1)—often called a point cloud—that can be used to generate three-dimensional representations of the Earth’s surface and its features. Many lidar systems operate in the near-infrared region of the electromagnetic spectrum, although some sensors also operate in the green band to penetrate water and detect bottom features. These bathymetric lidar systems can be used in areas with relatively clear water to measure seafloor elevations. Typically, lidar-derived elevations have absolute accuracies of about 6 to 12 inches (15 to 30 centimeters) for older data and 4 to 8 inches (10 to 20 centimeters) for more recent data; relative accuracies (e.g., heights of roofs, hills, banks, and dunes) are even better. The description of accuracy is an important aspect of lidar and will be covered in detail in the following sections.

The ability to “see under trees” is a recurring goal when acquiring elevation data using remote sensing data collected from above the Earth’s surface (e.g., airplanes or satellites). Most of the larger-scale elevation data sets have been generated using remote sensing technologies that cannot penetrate vegetation. Lidar is no exception; however, there are typically enough individual “points” that, even if only a small percentage of them reach the ground through the trees, there are usually enough to provide adequate coverage in forested areas. In effect, lidar is able to see through holes in the canopy of vegetation. Dense forests or areas with complete coverage (as in a rain forest), however, often have few “openings” and so have poor ground representation (i.e., all the points fall on trees and mid-canopy vegetation). A rule of thumb is that if you can look up and see the sky through the trees, then that location can be measured with lidar. For this reason, collecting lidar in “leaf-off” conditions is advantageous for measuring ground features in heavily forested areas.

Read it here