LIDAR is a remote sensing technology that stands for light detection and ranging which uses the pulse from a laser to collect data that helps to measure and create 3D models and maps of distant objects and environments.
A LIDAR system works in a way that is similar to radar and sonar but instead of sound or radio waves uses light waves from a laser. A LIDAR system uses “Time of Flight measurements which calculates the amount of time that the light takes to reach the object being studied and the time that it takes to reflect back to the scanner”. The data is then interpreted using velocity of light which is 299,792,458 meters per second. The rate of firing pulses in a LIDAR system is about 160,000/second. The data that is derived is then converted to a 3D visualization known as “point cloud”. The point density is about less than a meter and the vertical accuracy is about 15cm while the horizontal accuracy is about 40cm
LIDAR has now been integrated with various technology to give high-end 3D representation of target objects which has various applications in the fields of archaeology, atmospheric physics, airborne laser swath mapping (ALSM), forestry, geodesy, geomatics, geography, geology, geomorphology, laser guidance, laser altimetry, and seismology. With the emergence of newer technology LIDAR has found application in autonomous vehicles as well.
LIDAR with the help of a laser scanning system that is integrated with Inertia Measurement unit(IMU) and GNSS receiver measures each point in the resulting point cloud with georeferenced. Each point from the point cloud is then combined to create the 3D image of the target object or the area.
LIDAR maps gives both absolute and relative positional accuracy that allows the user to determine the location from where the data was taken and how each point relates to the object in terms of distance.
LIDAR maps can be used to map entire cities with accuracy up to millimeters of structures and areas therein. LIDAR maps are very helpful in extracting structural details and classifying objects such as bridges, road networks, vegetation and other structural details of a city. Comparative time images of an area can help substantially in understanding the changes and effects of environmental factors such as erosion, degradation forestation etc.
Components of LIDAR system
The Light Detection and Ranging system has 4 main parts that work together to produce highly accurate images of the objects or area being studied. They are:
- LIDAR Sensors: It fires the laser pulses as it scans the ground from side to side.
- Positioning and navigation system: GPS receivers tracks the altitude and accurate location of the plane. The Inertial measurement units or the IMU tracks the tilt of the airplane as it flies and fires the pulses. Elevation calculations are made using the tilt of the plane to measure the incident angle of the pulse.
- Scanners: The quality and speed of the image that is developed depends upon the speed at which they are scanned. The scanner includes a dual oscillating plane mirrors and a combination with polygon mirror and a dual axis scanner. A beam splitter is used to receive the return signal. The quality of optic choices used determines the angular resolution and range that can be detected.
- Computers: All the data that is collected is recorded and converted into images.
Outputs of LIDAR:
Number of Returns: LIDAR works by sending out pulses which traverses the same way as light rays. When the pulses are directed towards a forest, they are reflected back from various surfaces that it hits before hitting the ground. The pulses are reflected back by the leaves, branches and then by the ground. The number of returns of the pulses gives valuable information in understanding the structure of the forest and the shape of the trees.
- Digital elevation models: LIDAR is frequently used in the DEM of bare earth surface models. These models are derived from the last returns that are filtered out from the various returns of the pulses that hit the ground. The points are then interpolated to get the digital elevation model maps which can help in identifying the slopes with the degrees in rise and fall, slope direction and hill shade.
- Canopy Height Model: A canopy height model helps to generate the map with details of what are the structures on the ground like trees and buildings. The difference between the first and the last return is calculated to determine the true height of the topological features on the ground.
- Light Intensity: Based on the composition and the angle of the surface of the objects the strength or the intensity of the return pulses vary. The variation in the return LIDAR light intensity is useful indentifying not just the composition of the object surface but also helps to understand its structural features.
- Point Classification: The American Society for Photogrammetry and Remote Sensing (ASPRS) has listed out a set of classification codes for LIDAR. A single point classification may fall into more than one category in which case the point is assigned a secondary class.
Storage of the Return
The LIDAR pulse that is returned can be stored in two different ways. It can be stored either as Full waveform or as Discrete LIDAR.
When the pulses hit the various structures on the ground it is returned multiple times depending on the structure. For example, if it is tree then the top branches return the pulses first then it is returned by the mid and then by the lowest branches before being returned by the actual ground surface. When the data is recorded as separate returns it is called as Discrete return LIDAR. When the whole return data is recorded as a continuous wave then it is called as a Full Waveform LIDAR.
LIDAR data is stored in LAS file format as a point cloud maintained by ASPRS. The LAS format assures a secure exchange of data between vendors and customers.
Types of LIDAR Systems: There are four basic types of LIDAR system discussed below:
- Ground-based LIDAR: This type of LIDAR system is set on a tripod which scans the hemisphere. It is helpful in scanning buildings and other elevated structures. It is frequently used in geology, forestry and construction applications.
- Profiling LIDAR: This was one of the initial types of LIDAR system that was used in the 1980s to measure single line features. The Profiling LIDAR system sends out a single pulse at a time in one line. It measures elevation single transect with a single fixed Nadir angle.
- Large Footprint LIDAR: This type of LIDAR uses full waveforms and the returns are averaged in 20m footprints. This form of LIDAR is rarely used due to the difficulty in getting the terrain from a large footprint LIDAR return. Only two LIDAR systems with the large Footprint type; SLICER and LVIS has been made by NASA as an experiment.
- Small Footprint LIDAR: This is the common type of LIDAR system that is used today. It scans at about 20 degrees angle both backwards and forwards. There are two main types of Small footprint LIDAR; Topographic LIDAR which is used to map land surface and Bathymetric LIDAR which is used for mapping seafloor and riverbeds or any other water bodies.
Applications of LIDAR
- Geology and soil science: The study of topological maps using LIDAR technology helps in determining the land surface elevations not just on bare earth but beneath canopy and underwater as well. LIDAR technology has been effectively used in the studying the changes in land topology which helps the scientists to understand physical and chemical processes leading to landscape shapes. Slope changes and landform breaks helps to understand patterns in soil spatial relationships.
- Atmosphere: LIDAR technology has been used in a range of measurements like cloud profiling, studying aerosols, measuring atmospheric components and wind speed.
- Biology and conservation: LIDAR system allows the measurements of canopy heights and biomass. The varied reflective index also helps in determining the type of foliage available in an area. With a generation of maps and detection of varied foliage, conservation steps can be taken for particular region or a type of species.
- Autonomous vehicles: Self-driven or the autonomous vehicles use LIDAR technology to detect any obstacles or other vehicles in its vicinity. This helps the autonomous cars to avoid the obstacles and navigate safely.
- Archaeology: Archaeology uses LIDAR to help in planning field campaigns, create high-resolution DEMS of archaeological sites to reveal microtopography, Mapping land surface covered by vegetation and mapping a broad overview of an area which is not visible from the ground.
- Agriculture: Agricultural robots function on LIDAR technology to navigate around and analyze the yield rates on different areas of the farmland. This helps to ascertain the need of fertilizers and weed control. Many agricultural research organizations use the topological maps derived from a LIDAR system to detect the regions with optimal conditions for a crop type.
- Rock mechanics: The use of LIDAR in the study of rock mechanics helps in understanding the changes in the spacing, orientation, aperture, persistence, roughness and water infiltration in the rocks. This helps to get a better understanding of the slope changes and other geomechanical properties of the rocks.
- Military: The military have used LIDAR technology in many of its functions the study of terrains using topological maps of bare earth surface and that hidden for vegetation. This technology has been used to design the detection of biological threats and landmines to deter any mishaps. Unmanned vehicles or autonomous vehicles have been introduced in the military that functions on LIDAR technology.
- Law enforcement: LIDAR technology is used in the Speed gun that is used by the police to measure the speed of a vehicle. LIDAR also finds a significant use in forensic studies to detect bullet trajectory and crime scene scans.
- Solar photovoltaic deployment: LIDAR technology is used to map and identify the right spaces and location for setting up of Solar Photovoltaic systems.
- Wind farm optimization: LIDAR is being used in measuring the wind speed and wind turbulence to help in the increase of the energy output of the wind farm. The incident wind can be measured so that the blades can be adjusted to reduce damage and maximize the output.
- Forestry: LIDAR has helped in understanding the extent of the forest boundaries and the type and thickness of the vegetation. The data collected can help in determining the age of the trees, the species and other statistical information.
- Surveying: LIDAR is popular with companies in the remote sensing field to survey an area using the Digital elevation models and Digital Terrain Models that helps not just in ascertaining the ground elevation but the canopy height as well.
- Spaceflight: LIDAR is used for range-finding and orbital calculation of relative velocity for the purpose of proximity operations and station-keeping a spacecraft. LIDAR is also used in detecting and identifying particles in the atmosphere and the space.
- Robotics: LIDAR technology has been integrated with robotics to get high degree precision of environmental perception enhancing the performance and accuracy of its function.
- Mining: Robotic mining vehicles with LIDAR sensors are used in mines to navigate in the mines. Ore volumes is also calculated by determining the changes in the area of the ore removal
- Physics and Astronomy: LIDAR is frequently used in the field of atmospheric physics to determine the wind speed and the content of various elements in the higher strata of the atmosphere. LIDAR technology has been extensively used in the field of astronomy to study the surface of the moon and other planets like the mars.
LIDAR has been integrated with various other technologies to enrich the functionality and the end product. LIDAR has not only found a way into the various fields of study, but it is being actively incorporated into video games virtual graphics simulation to heighten the experience. LIDAR has been also used with the Structure from Motion Technologies which allows the delivery of 3D images and maps extracted from data derived from visual and IR photography. The high-resolution imagery and the its capacity to target a wide range of materials from large rocks to the smallest molecules has made LIDAR a powerful and popular technology in various fields.