Understanding LiDAR

Humans have a tendency to learn from nature. The primary source of evolution of LiDAR was in nature millions of years ago, Chiroptera which is commonly known as Bat uses a system guided by echolocation now widely known as SONAR (Sound Navigation And Ranging). Bats use echoes of self-produced sounds bouncing off objects to help them navigate.

The first form of such development in an artificial system is known as RADAR (Radio Detection And Ranging) which uses radio waves outside the audible range. it consisted of Antenna, Transmitter and Receiver. At first it was used to detect metallic objects and distance range of only about 3km, much less than today’s systems.

But, What is lidar?

Lidar is an acronym for Light Detection And Ranging (also written as LiDAR), this uses the same principles of RADAR but emits intense, focused beams of light and measures the time it takes for the reflections to be detected by the sensor.

lidar helps to get three-dimensional coordinates – X, Y Z (i.e. Latitude, Longitude and Altitude/ Elevation). These three coordinates are calculated using:

  • The time difference measured between the light pulse emitted from the source and returned from the target.
  • The angle at which the light pulse was emitted.
  • The absolute location of the sensor on the surface or above the earth.

What are lidar platforms?

There are three types of lidar platforms:

  1. Ground-based/ Terrestrial lidar Platform
  2. Airborne lidar Platform
  3. Spaceborne lidar Platform

1. Ground-based /Terrestrial lidar Platform:

One of the conventional methods that helps in accurate measurement of archaeological sites, buildings, different types of rock formation studies. terrestrial lidar is generally mounted on a stationary tripod.

The recent advancements in Terrestrial lidar show that great potential provides calibration/validation data for passive and active air/space-borne data. It also provides better quantified (and, generally, reduced) uncertainty and more traceability for traditional forest inventories.

Source: ​http://sites.bu.edu/lidar/

Terrestrial LiDAR Platform

2. Airborne Lidar Platform

The systems which are mounted on aircraft, helicopters and Unmanned Airborne Systems are called as airborne lidar. Some of the most widely used applications are topographic mapping, forest canopy mapping, etc.

There is a substantial increase in the use of UAS and LiDar have opened up wide possibilities to explore the terrain and get useful insights.

Airborne LiDAR Platform

Source: ​NOAA

3. Space borne Lidar Platform:

Spaceborne Lidar is the lidar operated at space platform, such as satellite, space station, space shuttle.

Spaceborne Lidar has used to do remote sensing the properties of earth’ s atmosphere, surface, ocean, etc.

In order to receive information from the earth, Spaceborne lidar needs to receive enough return power and enough ratio of signal to noise.

Space borne LiDAR Platform

Source:​​spie.org

Here is an interesting table which talks about the classification of LiDAR with respect to principal, values and target.

interesting table which talks about classification of LiDAR

What are the components of the lidar system?

components of the lidar system include the following :

1. Lasers:

The Lasers are categorized by their frequency and wavelength

  1. Wavelength:
      • infrared (1500 – 2000 nm) for meteorology – Doppler LiDAR
      • near-infrared (1040 – 1060 nm) for terrestrial mapping
      • blue-green (500 – 600 nm) for bathymetry
      • ultraviolet (250 nm) for meteorology

2. Scanners and Optics:

The scanner helps to measure the angle at which the pulse was fired. ​Angular resolution and range is affected by the choice of Optic

3. Global Positioning System (GPS) & Inertial Measurement Unit (IMU):

global positioning system (GPS) is required to get the location information (x, y, z) of the scanner.

IMU helps to identify the accurate location of the sensor at a given location. These components help to convert the sensor information into a statistical form which can be further utilized for various calculations.

4. Precise Clock:

The clock is one of the important components which calculates the timestamps at the time of the fire and at the time of reception.

In LiDAR system distance is calculated by:

Distance = (Speed of Light x Time of Flight) / 2

What is the basic return principle?

The number of returns after the incidence of objects on earth gives information about it. If the returns are more than one, it is called as multiple returns. In general, a system can capture up to 5 returns which can increase the amount of data generation by 30% or more hence, there is an increasing inability to look at the 3-dimensional surface containing above earth objects.

In the case of trees, the number of returns helps to understand canopy structure in depth.

multiple returns

Here, the figure shows drilled down returns from the top of the tree to its base. Each return is recorded with its X, Y, Z location and time travelled to and fro is taken into consideration.

Applications of lidar:

applications of lidar are listed down here:

1. Archaeological Applications:

The ability to provide high precision Digital Elevation Models (DEM) can help archaeologists to study the micro-level topography which might be hidden by vegetation otherwise. Easy integration with Geographical Information System (GIS) helps analysis and interpretations.

2. Forestry Application:

Vertical structure of canopies, bulk and density of forest areas can be measured and the ability of LiDAR to penetrate through the canopy which helps to predict the canopy base heights. Forest fuel calculation in terms of forest fire modelling.

3. Mapping and Topography:

LiDAR is especially suitable for mapping terrain models, including complex mountain topography. The 3D aspect of LiDAR makes high-resolution contour maps.

4. Self-driving cars:

To give an eye for self-driving cars, LiDAR is used in such a way that it gives a 360-degree view of surrounding objects which helps to identify close and far objects and assist in safe driving.

5. 3D City Modelling:

Huge applications in the field of 3D city modelling, making 3D city maps to walk through and give VR and AR effects are capturing the hearts of the new generation.

References:

  1. Terrestrial LiDAR for measuring above-ground biomass and forest structure K.Calders , P.Wileks et.al
  2. https://coast.noaa.gov/data/digitalcoast/pdf/lidar-101.pdf
  3. http://www.lidar-uk.com
  4. https://spie.org/news/2655-a-spaceborne-lidar-for-high-resolution-topograph ic-mapping-of-the-earths-surface?SSO=1

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