LiDAR is an acronym for light detection and ranging, an optical technology for distance detection. The first attempts to measure distances by light beams were first made in the 1930s, and airborne LiDAR became more commonly used in the 1960s, with geospatial measurements beginning in the 1980s.
As the technology has evolved, the uses of LiDAR have continued to expand. More recently, it has been used for remote sensing, 3D mapping and canopy height modeling. It has also proven effective and vital for autonomous vehicles and advanced driver assistance systems (ADAS).
What is LiDAR?
LiDAR is a distance measurement technology. It uses light like a pulsed laser to detect the distance between objects in an environment.
A laser source and receiver are key components of a LiDAR system. The source transmits laser pulses reflected from objects in the target scene. The receiver detects reflected laser pulses and measures the distance to objects in the scene based on time-of-flight (TOF) calculation.
Significantly, LiDAR can measure the distance between objects while capturing the environment. This is why it is effective for mapping and modeling surfaces.
As the system detects reflected laser pulses, it measures distances based on time of flight – known as “light detection and ranging.”
Understanding Remote Sensing
There are two types of LiDAR systems, aerial and ground-based LiDAR. Airborne LiDAR originated and became one of the common remote sensing methods available. It's even installed on helicopters and planes, sending light like pulsed lasers to the ground. The pulses reach the ground and return to the sensor. Then, the system measures how long it takes the light to return. Based on the return time, LiDAR measures the distance.
A LiDAR system can generate an accurate three-dimensional map detailing the shape and surface of the Earth using distance measurements, GPS data and/or other data recorded by the aerial system.
Remote sensing uses two types of LiDAR systems, topographic and bathymetric. Topographic LiDAR uses an infrared laser to map the Earth, and bathymetric LiDAR uses green light that penetrates water to map the seafloor and riverbed surfaces.
Airborne LiDAR is used for surface mapping of natural and man-made structures. For example, it is used to map geology, land surface and coastlines to generate digital elevation models or for land monitoring, urban planning and even natural disaster planning (to assess floods, landslides, tsunamis, etc. .). It is also commonly used to survey and monitor vegetation, agriculture, forestry, rivers, watersheds, and for archaeological research.
LiDAR can also measure and trap gases and particles in the atmosphere. In this way, it is also sometimes used for climate monitoring and meteorology.
How LiDAR works
Since LiDAR is a ranging device, the laser light, sensor and GPS are its most important components. The distance between objects in a given space is recorded by measuring the time lapse between transmitted and backscattered light pulses. The system generally uses a scanning mirror or multiple beams of laser light to scan the target area.
An ideal laser beam wavelength is used to generate digital surface models or digital elevation models – one that is safe for human eyes but will not be absorbed by atmospheric gases. These systems typically transmit about 160,000 laser pulses per second, scanning each meter pixel with about 15 pulses.
As a plane moves, for example, the LiDAR system scans the ground surface from one side to the other. It scans an area of 1 to 5 km. Most of the laser pulses are transmitted into the ground at an angle and the rest are directed below the system.
The transmitted pulses are then reflected from the ground or other objects. It has the same intensity whether it is reflected from the ground or the top of a canopy. But if the laser beam hits the branches of a tree or the edges of a building, it will be reflected several times before being received by the sensor. The scattered laser beams return with lower intensity. In these cases, multiple reflections (i.e., the first, second, third return, and so on) are recorded from a single laser pulse.
A LiDAR system records multiple data points simultaneously, including:
- Light intensity of detected pulses
- X, Y and Z location via GPS
- Orientation of an object (such as the plane in the sky) with the help of an internal unit of measurement.
As objects such as trees and buildings on the ground scatter and reflect laser rays differently and at different intensities, LiDAR uses these differences to accurately and accurately map the ground surface, the objects on the ground, and the distance to them.
Beam transmission and detection are controlled by software. Airborne LiDAR systems typically have a resolution of 40 cm horizontally and 15 cm vertically.
For climate monitoring and meteorology, LiDAR systems use specific wavelengths scattered, absorbed or re-emitted by molecules and particles in the atmosphere. By measuring the concentration of raindrops, for example, LiDAR systems can predict the distance of a storm or estimate predicted precipitation. In many weather systems, LiDAR systems incorporate the Doppler effect to measure atmospheric wind speed.
LiDAR uses scanning technology and therefore works well in advanced vehicles. LiDAR can create a 3D model of surrounding driving conditions, even when the scenery (traffic, construction, pedestrians, etc.) changes. The Doppler technique can also be used to estimate the speed of other vehicles.
LiDAR is similar to but different from radar and sonar. Radar uses radio frequencies and sonar uses sound waves to scan the environment. LiDAR uses LASER beams.
LiDAR data
LiDAR systems record the distribution of a light waveform and light intensity. There can be two ways to record a light waveform – as a discrete return or a full waveform.
- In discrete feedback, spikes or other prominent objects are recorded as points on the waveform curve. These discrete points are called returns and are recorded on a scale of 1 to 4 or more.
- In a complete waveform, continuous distributions of the light waveform are recorded rather than discrete points. Full-waveform LiDAR records the most information, so it is more complex and involves more computational data to process.
Regardless of the recording method, LiDAR data is often also processed using discrete points, known as “point classification”. It is available as a collection of points called a LiDAR point cloud, which may be available in .LAZ or .LAS format files.
In LiDAR point cloud, data attributes are described by metadata. These attributes include the X and Y locations, the elevation value (Z), and the recorded light intensity value. There may be additional data such as GPS and orientation. The point cloud is often processed to derive the classification. The classification may be “soil” or “soilless”, “soil” or “vegetation”, or classification into buildings and other infrastructure.
Airborne LiDAR is primarily used to create high-resolution digital elevation models. It creates digital surface models to record the elevation of buildings, trees, soil and other structures. Canopy height models are derived by subtracting the elevation of the ground from the elevation of trees, buildings, or other objects.
These models are used to predict the true height of Earth's topographic features. LiDAR can also be used to record vertical forest structures, such as to identify specific tree species.
In addition to point classification, LiDAR data can be used as a continuous record of light intensity. In this type of LiDAR data, the reflective percentages of the returned light waveform are recorded. The data is then used by object-based image classification or image classification.
Terrestrial LiDAR
Terrestrial LiDAR systems are generally installed on moving vehicles. These systems create three-dimensional models of roads, infrastructure, buildings and highways.
Handheld LiDAR systems are used for architectural scans to create models of the interior and exterior of buildings. Terrestrial LiDAR systems include two types, static and mobile LiDAR.
LiDAR Applications
Some of the popular applications of LiDAR are described below.
1. Mapping: LiDAR creates three-dimensional models of the Earth's surface. These models can be digital surface models or digital elevation models.
2. Oceanography: LiDAR is actively used to scan coastlines and sea surfaces, measuring the exact depth of the ocean. It can detect marine life, lost ships or biomass at sea.
3. Agriculture: Remote sensing is the main application of a LiDAR system and can monitor crop yield, seed dispersal and crop exploitation on agricultural land.
4. Archaeology: Mobile LiDAR systems examine archaeological structures and create three-dimensional models to conserve them digitally.
5. Military: LiDAR systems are commonly deployed in border areas to detect intrusions.
6. Architecture: Mobile LiDAR systems can inspect the interior and exterior of buildings and create 3D models. They are commonly used to develop floor plans and interior designs.
7. Space: LiDAR systems support the safe landing of lunar vehicles.
8. Natural disasters: LiDAR systems estimate the distance and speed of storms and map the risks of flooding, coastal erosion and carbon stocks in forests.
9. Automobiles: Small, low-range LIDAR is used in ADAS systems to scan a dynamic scene and navigate autonomous vehicles.
Using LiDAR with Arduino
LiDAR is something you can experience for yourself. Check out this tutorial to learn how LiDAR systems work with Arduino.
