How does LiDAR work ?
What is this laser technology making mapping possible through vegetation, in low-contrast or shadowy situations?
Light Detection and Ranging
Light Detection And Ranging (LiDAR) is a laser-based remote sensing technology. The idea behind LiDAR is quite simple: point a small laser at a surface and measure the time it takes the laser to return to its source.
This technology is used in geographical information systems (GIS) to produce a digital elevation model (DEM) or a digital terrain model (DTM) for 3D mapping.
LiDAR operating principle
- Emission of a laser pulse
- Record of the backscattered signal
- Distance measurement (Time of travel x speed of light)
- Retrieving plane position and altitude
- Computation of precise echo position
LiDAR for drones matches perfectly with:
- Small areas to fly over (<10 sq. km or 100 km linear)
- Mapping under vegetation
- Hard-to-access zones
- Data needed in near real-time or frequently
- Accuracy range required between 2.5 and 10 cm
How does LiDAR work?
You may have already heard about LiDAR but have no clue about this technology. You’ll learn in what follows the basic principles behind LiDAR. You’ll also discover several applications for 3D laser mapping with unmanned aerial vehicles (also known as UAV, UAS or drones).
1. Understanding how LiDAR works
Light Detection and Ranging (LiDAR) is a technology similar to radar, using laser instead of radio waves.
LiDAR principle is pretty easy to understand:
- it emits a laser pulse on a surface
- catches the reflected laser back to the LiDAR pulse source with sensors
- measures the time laser travelled
- calculates the distance from source with the formula “Distance = (Speed of light x Time elapsed) / 2
This process is repeated a million times by LiDAR instruments and ends up producing a complex map of the surveyed area known as a 3D point cloud.
2. Understanding how a LiDAR system is built
The equipment needed to measure a million distances from sensors to surface points is a LiDAR system. This advanced technology operates really fast as it is able to calculate the distance between LiDAR sensors and its target (as a reminder light speed is 300 000 kilometers per second). LiDAR systems integrate 3 main components whether they are mounted on automotive vehicles, aircraft or UAV:
1. Laser Scanner
LiDAR systems pulse a laser light from various mobile systems (automobiles, airplanes, drones…) through air and vegetation (aerial Laser) and even water (bathymetric Laser). A scanner receives the light back (echoes), measuring distances and angles. The scanning speed influences the number of points and echoes that are measured by a LiDAR system. The choice of optic and scanner influences greatly the resolution and the range in which you can operate the LiDAR system.
2. Navigation and positioning systems
Whether a LiDAR sensor is mounted on aircraft, car or UAS (unmanned aerial systems), it is crucial to determine the absolute position and orientation of the sensor to make sure data captured are useable data. Global Navigation Satellite Systems (GNSS) provide accurate geographical information regarding the position of the sensor (latitude, longitude, height) and an Inertial Measurement Unit (IMU) defines at this location the precise orientation of the sensor (pitch, roll, yaw). Data recorded by these 2 devices are then used to generate data into static points : the basis of the 3D mapping point cloud.
3. Computing technology
In order to make the most of the data : computation is required to make the LiDAR system work by defining precise echo position. It is required for on-flight data visualization or data post-processing as well to increase precision and accuracy delivered in the 3D mapping point cloud.
3. Defining a fit between your project needs and LiDAR specifications
Laser Scanner: What is the level of accuracy, level of precision, point density, range, swath that fit to your project needs ?
GNSS: Are the GNSS reference station (terrestrial) + GNSS receiver (moving) compatible with GNSS used (GPS, GLONASS, BEiDOU or Galileo) ? Do I need a ground station or not ?
Batteries: Are the batteries internal or external ? What is the autonomy required to cover the surface you want to map ?
Mounting: Can the LiDAR system be easily mounted on the aerial platform (aircraft, drone) or automotive platform (car) you use?
Datafile: What is the format of the generated data file ?
Data Post-processing: How easy is to use the data and deliver the best 3D mapping point cloud to your end customer ? Classification, colorization, DTM generation, orl ? What to do with the post-processed data ?
4. Discovering UAV LiDAR applications
Energies & Utilities: powerline survey to detect line sagging issues or to plan trimming activities
Mining: surface/volume calculation to optimize mine operations (stockpile, excavation) or decide mine extension
Construction & engineering: mapping to help leveling, planning and infrastructure optimization (roads, railways, bridges, pipelines, golf courses) or renovating after natural disasters, beach erosion survey to build emergency plan
Archaeology: mapping through the forest canopy to speed up discoveries
Forestry: mapping forests to optimize activities or help tree counting
Environmental research: measuring growth speed, disease spreading
5. Discovering UAV for LiDAR mapping
Find out more about DJI UAV for LiDAR mapping such as DJI M600 or DJI M300.
Choosing the right UAV for your next LiDAR surveys is not easy. Read more about how to choose your UAV to start your LiDAR operations.
Discover what makes a good UAV LiDAR integration.
