Lidar (Remote Sensing Technology)
Lidar, short for "light detection and ranging," is a remote sensing technology that employs laser light to measure distances and create detailed 3D models of various terrains and structures. By emitting pulses of light toward a target and measuring the time it takes for the light to return to the sensor, lidar can accurately assess the elevation of landforms, vegetation density, and even the height of man-made structures. This technology has a wide range of applications, including environmental monitoring, archaeology, and planetary exploration, such as mapping the surface of Mars.
Lidar systems can be mounted on various platforms, including airplanes, helicopters, ground vehicles, and stationary tripods, allowing for versatile usage in different settings. The method is particularly effective under clear weather conditions but may be hindered by clouds or rain due to the shorter wavelengths of light it utilizes. Since its inception in the 1960s, lidar has undergone significant advancements, including integration with GPS and inertial measurement units for even greater accuracy in data collection.
Overall, lidar represents a powerful tool for both scientific research and practical applications, aiding in the understanding of environmental changes, topographical mapping, and uncovering hidden features beneath dense vegetation.
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Lidar (Remote Sensing Technology)
Lidar (often spelled LIDAR or LiDAR) is a surveying method that uses laser light to measure distances. The word lidar is an abbreviation for "light detection and ranging" but is also a combination of the words light and radar. Lidar systems work by sending out pulses of light toward a target and then measuring how long it takes for those light pulses to be reflected back to the sensor. Lidar is similar to radar and sonar but uses light instead of radio waves or sound waves. Lidar is a remote sensing technology, meaning it obtains measurements or observations from afar.
Lidar has a wide range of applications, including mapping elevations across landscapes, determining the canopy height of forests, evaluating vegetation density, calculating the thickness of ice sheets, examining human-built structures, revealing archaeological features, tracking shoreline changes and assessing storm damage to beaches, studying clouds and particles in the atmosphere, and exploring other bodies in the solar system.
Brief History
The first use of lasers to conduct remote sensing observations came in the 1960s, soon after lasers were invented. Early applications of lidar included studying clouds and the composition of the atmosphere, measuring changes in glaciers, and creating high-resolution topographic maps of Earth's surface. Also known as laser ranging, lidar was employed by NASA's Apollo 15, 16, and 17 missions to map the surface of Earth's moon in the 1970s.
With the development of the Global Positioning System (GPS) in the 1980s, lidar units could be operated on airplanes, with GPS measurements accurately tracking the plane's altitude and position while taking observations. Increased accuracy in airborne observations came in the 1990s with the advance of inertial measurement units (IMUs), which measure the orientation (pitch, roll, and heading) of an airplane while it is in flight.
Lidar systems were flown aboard space shuttle missions to study Earth from space. Lidar instruments are included on numerous spacecraft in orbit around Earth and other objects in the solar system.
Overview
Lidar is a proven technique for obtaining highly accurate distances to objects to map elevation or other structures, both natural and manufactured. It is commonly used to determine the height of objects on the ground such as landmasses, bodies of water, vegetation, bridges, roads, dams, or buildings.
A lidar device works by sending out pulses of laser light toward a target and then recording the reflected light that returns to the lidar sensor. The lidar system also records the time it takes for a pulse of light to make the round trip from when it leaves the device to when it returns. The distance the light travels can be calculated using the speed of light (about 186,000 miles or 300,000 kilometers per second). Contemporary lidar systems emit tens of thousands to hundreds of thousands of pulses each second, producing detailed maps of targeted areas.
Lidar units are commonly mounted on airplanes or helicopters, from which they are able to survey large regions relatively quickly. However, lidar can also be used on a stationary platform on the ground (such as a tripod) or on a mobile vehicle (such as a car or boat). For airborne measurements, GPS and an IMU provide precise information about the altitude, position, and orientation of the aircraft to account for the changing location of the lidar sensor when the observations are made.
Lidar is similar to radar in that it emits energy toward a target to determine the object's distance. But unlike radar, which uses radio waves, lidar uses visible, ultraviolet (light with shorter wavelengths than blue visible light), or near-infrared light (light with wavelengths slightly longer than red visible light). The light used for lidar has shorter wavelengths than radio waves, allowing lidar systems to detect smaller objects and resolve smaller features than radar can. However, the shorter-wavelength light is more easily deflected by clouds, rain, or dense haze, meaning lidar is only effective in clear weather conditions, unlike radar.
Lidar has numerous scientific and practical applications. For example, lidar systems are being used to better understand the effects of climate change. Lidar has been used to monitor changes in glaciers and the thickness of the Greenland Ice Sheet. One of the earliest applications of lidar was to study Earth's atmosphere, including its composition and structure as well as clouds and aerosols (airborne particles), which remains an important topic of study. Lidar is helping to identify changes in shorelines from strong storms such as hurricanes and long-term erosion.
Ecologists use lidar to study forests around the world. High-resolution lidar observations can provide tree counts, determine the height of specific trees or an overall forest canopy, and reveal the vegetation density in a wooded area. Lidar can also pierce the forest canopy to reveal details hidden below the foliage. For example, in November of 2016, the US Geological Survey (USGS) reported that lidar observations helped to reveal previously unrecognized landslides and large areas of unstable terrain concealed beneath the dense forest cover of the western Columbia Gorge in Washington State. Other lidar surveys have uncovered archaeological ruins in the dense forests in places such as Central America, Cambodia, and even New England in the United States.
Lidar-based discoveries are not limited to Earth, however. While lidar (or laser altimeter) instruments have been installed on Earth-orbiting NASA satellites such as CALIPSO and ICESat, they have also been included on spacecraft sent to other solar system objects (including the Moon, Mars, Mercury, and an asteroid) to map or measure the elevations on their surfaces. For example, the Mars Orbiting Laser Altimeter aboard NASA's Mars Global Surveyor spacecraft used lidar to gather elevation data over three years and precisely map the Martian surface. Another lidar instrument on NASA's Phoenix spacecraft, which landed on Mars, detected snow falling from the Martian sky. Lidar instruments may even help future planetary spacecraft gauge their distance above a planetary surface as they attempt to land on other worlds in the solar system.
Bibliography
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