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Image Credit: Oldrich Barak
Light Detection and Ranging – or LiDAR – is an active remote surveying method which has been used to examine the surface of the Earth and even map the exterior of the Moon. It uses pulsed laser light to measure the distance to an object and create a three-dimensional representation.
The technique originated in the 1960s in meteorology when it was mounted to aircraft but it wasn’t until the advent of a commercially viable GPS systems in 1980s that LiDAR became a useful means of providing precise geospatial measurements. Since then it’s been used in geology, archaeology and seismology to name a few applications: it’s even been used in space – Apollo 15 used a laser altimeter to map the surface of the Moon in 1971.
LiDAR makes use of ultraviolet (UV), visible or near infrared (IR) light to image objects and map their physical features with very high resolutions. Light from a laser is fired rapidly – up to 150,000 pulses a second - at the object and then reflected back to a sensor. The sensor measures the time taken for the pulse to bounce back and calculates the distance to the target with high accuracy. Differences in the laser return times and wavelengths can then be utilised to create a 3D map of the target.
A LiDAR system consists of four components: a laser, a scanner and optics, photodetector and receiver electronics, and navigation and positioning systems – particularly important in airborne LiDAR to ensure integrity. There are two main laser pulse models. The micropulse is common in computers and carries few safety warnings. High energy systems are used in atmospheric research for studying the layering and density of clouds, for example, or examining the properties of temperature and pressure.
Scanners and optics are an integral part of the system, with the type of optic determining the resolution and range that the system can detect. The speed at which images can be obtained is affected by how fast it can be scanned into the system and there are a variety of methods available depending on what is going to be perused: azimuth and elevation, dual oscillating plane mirrors and polygonal mirrors and dual axis scanners.
LiDAR is popular in surveying the built environment, so buildings, road networks and railways, and in creating digital models of terrain (DTM) or elevation (DEM) of specific landscapes. It’s been employed by archaeologists to find subtle variations in the ground that could point to hidden relics. For example, square patterns found on the ground over vegetation turned out to be ancient buildings and pyramids built by the ancient Mayan and Egyptian civilisations.
Environmental applications of LiDAR include mapping flood risk, carbon stocks in forestry and monitoring coastal erosion. The National Oceanic and Atmospheric Administration in the USA use LiDAR scanning to produce accurate shoreline maps, to create digital elevation models for use in geographic information systems and to assist in emergency response operations. They employ two types of scanning – topographical which used near infrared lasers to map the land – and bathymetric, which uses green light capable of penetrating water to measure the seafloor and river bed elevations.
But that’s not all, smaller, lower range LiDAR scanners are used in driverless cars where they are considered the most important piece of hardware. Google and Uber cars have bulky boxes on top – the LiDAR system - which fire laser pulses continuously to give a 360° visibility and precise information about the area around them.
The system creates a 3D map for the car to navigate predictably through. Google’s cars are capable of detecting pedestrians and cyclists, stop signs and other obstacles, and as LiDAR becomes operational at higher resolutions and at longer ranges it will be used to detect and track objects, differentiating between someone on a bike or walking, taking into account their speed and direction.
Sources and Further Reading
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