14 LiDAR BASIC PRINCIPLES AND APPLICATIONS

Swati katiyar

Learning Outcome

•  Student will get to know the need of LIDAR over optical and microwave.
• Student will acquire skill how to study and work upon Lidar data.
•  Student will be equipped with knowledge to study further about applications of LIDAR and its advancements in remote sensing fields.

Introduction

LiDAR is acronym for Light Detection and Ranging. It provides very accurate, high resolution 3D data. Captured using special sensors, from the air or the ground, it results in a set of “dots” suspended in a three-dimensional space. These dots can be displayed in special software or converted into a 3D mesh for use in many modern 3D software packages, such as 3D Studio MAX, Maya and Sketchup.

LiDAR technology uses light sensors to measure the distance between the sensor and the target object (Figure 1). From an aircraft this includes objects such as the ground, buildings and vegetation. For ground based LiDAR it measures building fronts and street furniture in extreme detail. With the latest technologies it is also possible to obtain colour values of the scanned surface to create an automatically texturered model.

LiDAR is ideal when very high accuracy measurements are required and is very cost effective for the amount of data generated. Airborne LiDAR is becoming more and more popular as a source of terrain mapping due to the high levels of detail it provides. Many companies now offer substantial amounts of “off-the-shelf” data as new areas are being flown and added to existing archives.

Ground-based LiDAR, which records “street scenes”, has been around for several years however only now is it beginning to become more common with off-the-shelf data more prevalent.

Many enthusiasts have created home-made LiDAR scanners, from simple devices such as distance measuring lasers to more complex 3D laser scanners similar to many professionally manufactured models. A quick search on YouTube gives an example of the technologies being developed.

Figure 1 LiDAR technology

Source: http://vcgi.vermont.gov/LiDAR

A brief history of LiDAR

The oldest known variation of modern LiDAR systems evolved in nature millions of years ago. Chiroptera, more commonly known as the bat, uses an echolocation guidance system now known as SONAR (SOund Navigation And Ranging). They emit short, loud ‘chirps’ from their noses and receive an echo through their ears in the form of two antennae. This provides the bat with a three-dimensional view of the surrounding area, allowing them to avoid obstacles and easily find their prey.

Humans started to develop similar systems in the beginning of the 20th century. Christian Huelsmeyer’s “Telemobiloscope”, developed in 1904, was the first form of RADAR (RAdio Detection And Ranging) sensor. This used radio waves outside the audible range. It consisted of an antenna, a receiver and a transmitter. Its original use was to detect metallic objects, in particular ships at sea, as a form of collision avoidance. This early form of RADAR had a distance of 3000m, much less than today’s modern alternatives. On the detection of an object it would sound a bell until the object left its path.

The method of determining distance was later solved by aiming the beam at any level of elevation. Taking into account the height of the transmitting antenna and the angle of vertical elevation of the detected object allowed simple calculation to determine the distance of the object from the transmitter. RADAR transmit a narrow, rectangular shaped pulse modulating in a sine wave carrier. Distance is measured by the time it takes the pulse to travel to and from the target. It is also possible to use a continuous waveform showing the Doppler frequency shift to measure the targets velocity.

The Doppler Effect: The Doppler effect is named after Christian Andreas Doppler (1803-1853). Doppler was an Austrian mathematician and physicist. He was born in Salzburg, Austria, the son of a stone mason. After completing high school Doppler studied astronomy and mathematics in Vienna and Salzburg and started work at the Prague Polytechnic. At the age of 39 Doppler published his most famous work, “Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels” (On the colored light of the binary stars and some other stars of the heavens). In this works Doppler suggested his principle that the observed frequency of a wave depends on the relative speed of the source and the observer. He tried to use this theory for explaining the colors of binary stars.

LiDAR (Light Detection And Ranging) sensors work on the same principle as RADAR, firing a wavelength at an object and timing the delay in its return to the source to measure the distance between the two points. Because laser light has a much shorter wavelength it is possible to accurately measure much smaller objects, such as aerosols and cloud particles, which makes it especially suitable for airborne terrain mapping.

The first optical LASER was built in 1960 by Hughes Aircraft, Inc. Laser instrument were soon used to compute distance by measuring the travel time of light from laser transmitter to a target and then back to laser receiver. Early remote sensing LiDAR systems could only collect measurements directly underneath the aircraft, creating a single profile of elevation measurements across the landscape. The synergistic use of kinematic GPS and inertial measurement (IMU) on airborne LiDAR scanning systems has allowed the technology to mature rapidly. LiDAR derived horizontal and vertical accuracies and cost of operation are now similar to that of photogrammetry.

LiDAR has been used extensively for atmospheric research and meteorology due to its excellent resolution. It was only with the deployment of Global Positioning Systems (GPS) in the 1980s, allowing the precise positioning of aircraft, that made airborne LiDAR surveying possible. Since then many downward looking LiDAR instruments have been developed for aircraft and satellite use.

How does LiDAR works

The principle behind LiDAR is really quite simple. Shine a small light at a surface and measure the time it takes to return to its source. When you shine a torch on a surface what you are actually seeing is the light being reflected and returning to your retina. Light travels very fast – about 300,000 kilometers per second, 186,000 miles per second or 0.3 metres per nanosecond so turning a light on appears to be instantaneous. The equipment required to measure this needs to operate extremely fast. Only with the advancements in modern computing technology has this become possible.

The actual calculation for measuring how far a returning light photon has travelled to and from an object is quite simple:

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

The LiDAR instrument fires rapid pulses of laser light at a surface, some at up to 150,000 pulses per second. A sensor on the instrument measures the amount of time it takes for each pulse to bounce back. Light moves at a constant and known speed so the LiDAR instrument can calculate the distance between itself and the target with high accuracy. By repeating this in quick succession the instrument builds up a complex ‘map’ of the surface it is measuring. With airborne LiDAR other data must be collected to ensure accuracy. As the sensor is moving height, location and orientation of the instrument must be included to determine the position of the laser pulse at the time of sending and the time of return. This extra information is crucial to the data’s integrity. With ground based LiDAR a single GPS location can be added for each location where the instrument is set up.

Generally there are two types of LiDAR detection methods. Direct energy detection, also known as incoherent, and Coherent detection. Coherent systems are best for Doppler or phase sensitive measurements and generally use Optical heterodyne detection. This allows them to operate at much lower power but has the expense of more complex transceiver requirements. In both types of LiDAR there are two main pulse models: micropulse and high-energy systems. Micropulse systems have developed as a result of more powerful computers with greater computational capabilities. These lasers are lower powered and are classed as ‘eye-safe’ allowing them to be used with little safety precautions. High energy systems are more commonly used for atmospheric research where they are often used for measuring a variety of atmospheric parameters such as the height, layering and density of clouds, cloud particles properties, temperature, pressure, wind, humidity and trace gas concentration. Working principle of LiDAR shown below Figure 2

Figure 2 Working principle of LiDAR

Source: https://www.slideshare.net/akshrana/LiDAR-56626481

Most LiDAR systems use four main components:

Lasers:

Lasers are categorised by their wavelength. 600-1000nm lasers are more commonly used for non-scientific purposes but, as they can be focused and easily absorbed by the eye, the maximum power has to be limited to make them ‘eye-safe’. Lasers with a wavelength of 1550nm are a common alternative as they are not focused by the eye and are ‘eye-safe’ at much higher power levels. These wavelengths are used for longer range and lower accuracy purposes. Another advantage of 1550nm wavelengths is that they do not show under night-vision goggles and are therefore well suited to military applications.

Airborne LiDAR systems use 1064nm diode pumped YAG lasers whilst Bathymetric systems use 532nm double diode pumped YAG lasers which penetrate water with much less attenuation than the airborne 1064nm version. Better resolution can be achieved with shorter pulses provided the receiver detector and electronics have sufficient bandwidth to cope with the increased data flow.

Scanners and Optics:

The speed at which images can be developed is affected by the speed at which it can be scanned into the system. A variety of scanning methods are available for different purposes such as azimuth and elevation, dual oscillating plane mirrors, dual axis scanner and polygonal mirrors. They type of optic determines the resolution and range that can be detected by a system.

Photodetector and receiver electronics:

   The photodetector is the device that reads and records the signal being returned to the system. There are two main types of photodetector technologies, solid state detectors, such as silicon avalanche photodiodes and photomultipliers.

Navigation and positioning systems:

When a LiDAR sensor is mounted on a mobile platform such as satellites, airplanes or automobiles, it is necessary to determine the absolute position and the orientation of the sensor to retain useable data. Global Positioning Systems provide accurate geographical information regarding the position of the sensor and an Inertia Measurement Unit (IMU) records the precise orientation of the sensor at that location. These two devices provide the method for translating sensor data into static points for use in a variety of systems.

LiDAR Laser and Scanning System

The LiDAR instruments consist of a system controller and a transmitter and receiver. As the aircraft moves forward along the line of flight, a scanning mirror directs pulses of laser light across track perpendicular to the line of flight (Figure 3a). Most LiDAR used for topographic mapping use eye safe near infrared laser light in the region from 1040 to 1060nm. Blue green lasers centered at approximately 532nm are used for bathymetric mapping due to their water penetration capability. LiDAR data can be collected at night if necessary because it is an active system, not dependent on solar illumination. LiDAR systems can emit pulses at rates >100,000 pulses per sec, often referred to as pulse repetition frequency. A pulse of laser light travels at c, the speed of light. LiDAR technology is based on the accurate measurement of the laser pulse travel time from the transmitter to the target and back to receiver. The travelling time of a pulse of light, t, is:

The range measurement process results in the collection of elevation data points (generally referred as masspoints) arranged systematically in time across the flightline (Figure 3b). The example displays masspoints associated with the ground, several powerlines, a pole and tree canopy.

Figure 3a LiDAR data collection                  3b LiDAR elevation masspoints for small area

Source: (3a) http://www.asu.edu/courses/art345/pike_b/terrainmapping/LiDAR.html

(3b) Remote Sensing of the Environment (second Edition), John.R.Jensen

Types of LiDAR

LiDAR, or 3D laser scanning, was conceived in the 1960s for submarine detection from aircraft and early models were used successfully in the early 1970’s in the US, Canada and Australia. Over the past ten years there has been a proliferation in the use of LiDAR sensors in the United Kingdom, with several regularly used in both airborne and ground surveying. This has been accompanied by an increase in the awareness and understanding of LiDAR in previously unrelated industries as the application of LiDAR has been adopted.

Airborne LiDAR:

Most airborne LiDAR systems are made up of the LiDAR sensor, a GPS receiver, an inertial measurement unit (IMU), an onboard computer and data storage devices.

The LiDAR system pulses a laser beam onto a mirror and projects it downward from an airborne platform, usually a fixed-wing airplane or a helicopter (Figure 4). The beam is scanned from side to side as the aircraft flies over the survey area, measuring between 20,000 to 150,000 points per second. When the laser beam hits an object it is reflected back to the mirror. The time interval between the pulse leaving the airborne platform and its return to the LiDAR sensor is measured. Following the LiDAR mission, the data is post-processed and the LiDAR time-interval measurements from the pulse being sent to the return pulse being received are converted to distance and corrected to the aircraft’s onboard GPS receiver, IMU, and ground-based GPS stations. The GPS accurately determines the aircraft’s position in terms of latitude; longitude and altitude which are also known as the x, y and z coordinates. The LiDAR sensor collects a huge amount of data and a single survey can easily generate billions of points totaling several terabytes.

An IMU is used to determine the attitude of the aircraft as the sensor is taking measurements. These are recorded in degrees to an extremely high accuracy in all three dimensions as roll, pitch and yaw – the vertical and horizontal movements of the aircraft in flight. From these two datasets the laser beams exit geometry is calculated relative to the Earth’s surface coordinates to a very high accuracy.

The initial LiDAR data can be further enhanced using additional post-processing, some of which can be automated and some are manual. Further processing utilizes the multiple return signals from each laser pulse. By evaluating the time differences between the multiple return signals the post-processing system can differentiate between buildings and other structures, vegetation, and the ground surface. This process is used to remove surface features to produce bare earth models (DTM) and other enhanced data products.

It is also possible to do selective feature extraction, for example, the removal of trees and other vegetation to leave just the buildings.

Figure 4 Airborne LiDAR

Source: http://stormwise.uconn.edu/LiDAR/

Ground-based LiDAR:

Ground-based LiDAR systems are very similar, only that an IMU is not required as the LiDAR is usually mounted on a tripod which the LiDAR sensor rotates 360 degress around. The pulsed laser beam is reflected from objects such as building fronts, lamp posts, vegetation, cars and even people.

The return pulses are recorded and the distance between the sensor and the object is calculated.

The data produced is in a ‘point cloud’ format, which is a 3-dimensional array of points, each having x, y and z positions relative to a chosen coordinate system.

The structure of ground based LiDAR is shown below (Figure 5):

Figure 5 Ground based LiDAR

Source: http://sites.bu.edu/LiDAR/

Application of Remote Sensing in LiDAR:

1) Forest Planning and Management: LiDAR is widely used in the forest industry to plan and mange (Figure 6). It is used to measure vertical structure of forest canopy and also used to measure and understand canopy bulk density and canopy base height. Other uses of the LiDAR in the forest industry are the measurement of the peak height to estimate its root expansion.

Riaño et al. (2002) identified using Lidar the height and tree cover, height and canopy cover, crown base height and crown bulk density. First generated a digital terrain model by differentiation of laser pulses with lower height and later interpolated using the spline function. Thus, the vegetation height was estimated by removing the ground elevation above sea level. Depending on the individual pulses height, by an algorithm based on a cluster analysis differed the variables: tree height, crown base height and shrub height, and classified the pulses differentiating in trees, shrubs and ground. Crown bulk density was obtained by dividing crown foliage biomass over crown volume. The crown biomass was modeled using empirical specific equations for its estimating and the crown volume was calculated directly (volume between tree height and crown base height).

Figure 6 Ability of lidar to penetrate to the ground through gaps in tree canopies Source: https://www.e-education.psu.edu/geog481/node/2004

2) Forest Fire Management: LiDAR is becoming widely popular in forest fire management. Fire department is transforming from reactive to proactive fire management. LiDAR image helps to monitor the possible fire area which is called fuel mapping (fire behavior model).

In fire management, DTMs can be used as topographic inputs or as base elevation maps, which can be subtracted from canopy and vegetation heights to assess fuels. Topographic information such as slope, elevation, and aspect can be used as direct inputs into decision support systems such as FARSITE (Fire Area Simulator) and BEHAVE (Fire behavior prediction and fuel modeling system) (Morsdorf et al. 2004). These inputs are essential to successful fire behavior prediction modeling (Figure 7). Digital terrain models also provide the base elevation which is subtracted from digital surface models (DSMs) to estimate vegetation heights and fuel loading (Andersen et 2005, Lefsky et al. 2002, Morsdorf et al. 2004, Clark et al. 2004). ere are many current uses for high-resolution topographic Lidar information. Lidar-derived DTMs can be extremely detailed with absolute positional accuracies below 15cm in the vertical dimension and 50cm in the horizontal dimension (Morsdorf et al. 2004).

Figure 7 Hillshade derived from the 2014 LiDAR digital terrain model. (Credit: Benjamin Jones / U.S. Geological Survey)

Source: http://www.fondriest.com/news/landscape-changes-after-alaskas-anaktuvuk-river-fire.htm

3) River Survey: Water penetration green light (532 nanometers) of the LiDAR is used to measure under water. Under water information is required to understand depth, flow strength, width of the river and more . For the river engineering, its cross section data is extracted from LiDAR data (DEM) to create a river model, which will create flood and flood fringe map. In same way to understand sea under world, LiDAR data is used by the marine engineer (Figure 8). LiDAR provides very accurate information. River is very sensitive and few meter of change in information can bring disastrous or loss of properties. So LiDAR is used to create high resolution and accurate surface model of the river. These extracted LiDAR information can be used for the 3D simulation for better planning of the structures or buildings on the river bank.

Figure 8 River survey using LiDAR

Source: https://www.researchgate.net/publication/237564890_Integrating_Bathymetric_Topograph ic_and_LiDAR_Surveys_of_the_Colorado_River_in_Grand_Canyon_to_Assess_the_Effe ct_of_a_Flow_Experiment_From_Glen_Canyon_Dam_on_the_Colorado_River_Ecosyste m/figures?lo=1

4) Management of Coastline: LiDAR data of the coastline surface and under the water surface can be combined by researches to analyze the waves behavior and area covered by them. If these data are captured periodically then marine scientist can understand the coastline erosion occurrence.

5) Transport Planning: LiDAR data for road helps engineer to understand it and give a roadmap for the building it (Figure 9). As LiDAR are highly accurate technology it helps to understand width, elevation and length of the existing road. Road engineer use LiDAR data for below things as well:

Calculate Cut & fill, culvert sizing, vegetation removal, grade calculations and more. Height clearances

Right of way and surface conditions

Figure 9 Transportation planning

Source: https://quantumspatial.com/our-solutions/transportation

LiDAR technology uses the near-infrared band of the electromagnetic spectrum and measures the time it takes for a laser pulse to travel from the transmitter to the target and back to the receiver. Because the light speed is known, the distance can be calculated. An accurate timing system is needed to guarantee the resolution, because the laser pulses are sent at 3,000 to 10,000 times or more per second. The aircraft positioning is recorded using an inertial navigation unit associated with avionics systems, a high-accuracy Global Positioning System (GPS) receiver in the aircraft, and GPS base stations installed in known locations. Thus, it’s possible to determine 3-D georeferenced coordinates for each pulse and then correct the aircraft positioning in terms of roll, pitch and heading, thereby improving the system’s accuracy. Modern LiDAR systems can operate at a laser pulse repetitive frequency of 50-100 kHz per second. The density of the ground points depends on the aircraft elevation, the number of pulses per time, the scanner angle and the aircraft speed. To cover dense areas, multiples flights can be combined. For high-resolution topographic data, a LiDAR mission is flown at low altitude. After the field data collection process is completed, the data points already in the digital format are easily loaded in computer stations for processing and interpretation. Airborne LiDAR technology presents several advantages compared with traditional methods such as photogrammetry and total station ground survey. Data collection using the field survey can produce accurate information. However, this method demands a team to measure distances and angles in the field and is thus time consuming and expensive. Photogrammetry uses stereoscopic analysis of aerial photos to generate a digital elevation model (DEM) in the post-processing step, which can also be time consuming. Using LiDAR, time consumption can be reduced considerably because of the high data collection speed—up to 81 square kilometers or 20,000 acres per day, and the data are collected and stored digitally.

6) Oil and Gas Exploration: As LiDAR wavelengths are shorter, it can be used to detect molecules content in the atmosphere that has same or bigger wavelength. There is the new technology called DIAL (Differential Absorption LiDAR) which is used to trace amount of gases above the hydrocarbon region. This tracking helps to find the Oil and Gas deposits (Figure 10).

Figure 10 Oil and Gas Exploration using LiDAR

Source: http://explorationmapping.com/

Kincaid oil field is in Christian County, Illinois. Christian County is located on the southern flank of the Sangamon arch and has had recent oil development. Wells are mostly Silurian in age with some Devonian production. A LiDAR survey was flown between 12/2014 and 03/2015 by Digital Aerial Solutions using a Leica ALS70. Slope angle maps using ArcGis 10.3 were created from a terrain data set. 12 divisions were used with quantile classification and a green to red color ramp. Green is lower slope and red is higher slope. Quantile places an equal amount of data in each division and results in a good signature of the higher slope angles in relatively low slope areas. Natural breaks or “Jenks” classification is more appropriate for areas of high slope angles. As evident in Figure 3 much of the Silurian oil wells are in areas of red (higher slope)

7) Archeology: LiDAR has played important part for the archeologist to understand the As LiDAR can detect micro topography that is hidden by vegetation which helps archeologist to understand the surface (Figure 11). DEM created from LiDAR is feed into GIS system and it is combined with other layer for analysis and interpretation.

Figure 11 LiDAR technology in archeology

Source: http://www.theverge.com/2013/6/20/4445568/lasers-LiDAR-archaeology-detailed-topographical-maps

From 2012 to 2015, archaeologist Damian Evans and his team used Lidar technology, mounted on helicopters, to map some 2,230km² with an accuracy of +/- 150mm. With 16 data points measured every square metre, the researchers were not only able to pinpoint well-known monumental stone structures in exquisite detail, they also discovered the massive urban cultures which surrounded these temples, identifiable by the remains of earthworks such as mounds, canals, roads and quarries.

8) Solar Energy Planning: Solar energy are getting popular for heating and electricity purpose. Solar panels are used to absorb the heat energy from the sun and it is converted to heat or electricity energy. For the installation of the Solar panel there are some basic requirements which are identified by the help of the LiDAR data. Like Solar panel should be kept to south facing of the roof and it should have minimum area and so on

9)  Glacier Volume Changes: LiDAR is used to calculate the glacier change over the period. LiDAR image are taken in time series to see the change happening. For example, LiDAR image was taken of Iceland from 2007-2009 and project was completed on 2012. These captured data will help scientist to know the amount of volume change.

The least squares 3D Surface Matching Method is put forward by Armin Gruen for the problem statement of surface patch matching and its solution method in photogrammetry in 1985 . Based on the Generalized Gauss-Markoff model, the proposed method estimates the seven transformation parameters among different surfaces and minimizes the sum of squares of the Euclidean distances. It has been widely used among terrain change monitoring, commercial measurement and Photogrammetry. Based on this method, Pauline Miller (2009) also successes in acquiring glacier volume change results in Slakbreen district in Norway, using ASTER data and LIDAR data. Using light detection and ranging (LiDAR) data collected from surveys over six glaciers in Greenland and Antarctica, particle image velocimetry (PIV) was applied to temporally-spaced point clouds to detect and measure surface motion. The type and distribution of surface features, surface roughness, and spatial and temporal resolution of the data were all found to be important factors, which limited the use of PIV to four of the original six glaciers. The PIV results were found to be in good agreement with other, widely accepted, measurement techniques, including manual tracking and GPS, and offered a comprehensive distribution of velocity data points across glacier surfaces. For three glaciers in Taylor Valley, Antarctica, average velocities ranged from 0.8–2.1 m/year. For one glacier in Greenland, the average velocity was 22.1 m/day (8067 m/year).

10) Gaming: LiDAR technology is used to capture the surrounding area and this data is feed into the computer and color code is added to it. For example for the race track game, LiDAR will be used to capture the view of the real race track. This captured race track data will be used for the game.

11) Recording of Building: Ground based LiDAR can be used to record the inside of the It can be used to record the interior design too. This extracted data can be printed on the 3D printer to model it. Or when building is rebuild this recorded information can be used to restore the interior design.

12) Mining: LiDAR is also used in the mining business in various task. It is used to measure the ore volume by taking series of photos of ore extraction space. These interval photos are used to calculate the volume.

13) Tunnel Surveying: LiDAR is used to measure accurate and detailed measurements, used for analysis, assessment and modeling of the tunnel that is for railway track or road. This might be in the mountain, land or underwater.