LiDAR is a remote sensing method that uses laser to measure the elevation of surfaces like the ground, forest and buildings making use of UV, visible or near infrared sources to sense objects. It creates fast and accurate terrain models for application across different types of industries. It makes use of a scanning laser combined with GPS and Inertial technology to form a three dimensional set of points.
LiDar works on a simple principle. It measures the time light takes to travel from a source to an object and from the object to the source. You see an object because of the reflection of the light from the object that reaches the retina. Light travels at a speed of 300,000 kms/sec which is 186,000 miles per second. And the distance of the object is calculated as half the product of the speed of light and the time of flight .
Distanance = (Speed of Light x Time of Flight) / 2
The energy emitted by LiDAR system is known as ‘Pulse’ and the light reflected from the object is known as ‘Return’.
The system sends a pulse of light and it waits for the pulse to return and the distance of the system is calculated using the time taken for the emitted pulse to return back to the sensor. In the end, it gets variable distance to the objects. Infact this is how LiDAR gets its name from the mechanism it uses - Light Detection and Ranging.
While Sonar uses sound waves to map things , Radar uses radio waves , LiDAR on the other hand, uses light sent out from a laser. A LiDAR unit scans the ground from side to side while some pulses will be directly at nadir, most pulses travel at an angle. Actually, the scan angle affects the laser spot size and therefore the resolution. Therefore, the angle is kept small. The elevation is derived from the measure of the distance of the object from the sensor. Smaller the distance means higher elevation.
Airplanes, UAVs and helicopters are the most commonly used platforms for acquiring LiDAR data over broad areas and the data are collected basically from three sources
- From ground
- From Plane
- From satellites using Altimeters
The Different Types of LiDAR
There are three basic types of LiDAR which are :
Topographic LiDAR- This type of LiDAR is used to map the land and typically uses near-infrared light.
Bathymetric LiDAR- This uses water-penetrating green light to assess seafloor and riverbed elevations.
Terrestrial Lidar is used for mapping buildings, natural features, trees at ground level. It is also very useful for 3D models of heritage sites.
The Working of LiDAR
There four parts of LiDAR are LiDAR sensors, GPS receivers, Inertial Measurement Units (IMU) and computers.
- LiDAR sensor is mounted on a mobile platform such as satellites, airplanes or automobiles, to determine the absolute position and the orientation of the sensor to retain useable data. They scan the ground from side to side with a pulsed laser beam .The sensor has a detector which senses the returns from objects
- Global Positioning Systems provide accurate geographical information regarding the position of the sensor.They are used to measure the altitude and location. These variables are crucial in determining the accuracy of terrain elevation values.
- Inertial Measurement Unit (IMU) records the precise orientation of the sensor at that location. Elevation calculations use orientation to accurately determine the actual position of the pulse on the ground.
- Computers record the height information as the LiDAR sensor scans the surface.
These 4 parts of the LiDAR system work together to produce highly accurate and usable results. LiDAR is used for wide area mapping, coastal changes, floodplain mapping, engineering applications like map bridges, roads. They find wide applications in seismology, archeology , soil testing etc..
LiDAR systems allow us to examine both natural and man made environments with accuracy, precision, and flexibility.