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Light Detection And Ranging
Article · March 2016
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Syrian Arab Republic
Higher Institute for Applied Sciences and Technology
Communications Department
th
4 year
SEMINAR
LIDAR
Submitted by: Ali Assi
Supervisor: Dr.Nizar Zarka
Linguistic Supervisor: Mr. Fahmi Alammareen
Scientific Supervisor: Mrs. Ihsan Alhassani
Light
Detection and
Ranging
LIDAR
[4TH YEAR, SEMINAR]
Contents
A.
List of Figures ................................................................................................................... 2
B.
List of Abbreviation ......................................................................................................... 3
C.
Key Words........................................................................................................................ 4
D.
Abstract ........................................................................................................................... 5
2
Introduction:............................................................................................................................ 6
3
Entrance for LIDAR .................................................................................................................. 7
3.1
What is LIDAR? ................................................................................................................ 7
3.2
LIDAR Component ........................................................................................................... 7
3.3
The basic principle of LIDAR ............................................................................................ 9
4
Types of LIDAR Systems ......................................................................................................... 10
4.1
LIDAR platforms ............................................................................................................. 10
4.2
The Source of the Energy .............................................................................................. 11
4.3
The Target Object .......................................................................................................... 11
5
How the LIDAR identifies the different objects ..................................................................... 13
6
Effective Parameters Influencing Intensity Measurements .................................................. 16
A.
Environmental Effects ................................................................................................... 16
B.
Target surface characteristics........................................................................................ 16
C.
Instrumental Effects ...................................................................................................... 17
D.
The Measurement Geometry ........................................................................................ 17
6.2
Effective factors influencing bathymetric LIDAR intensity measurements ................... 18
7
Correction Methods .............................................................................................................. 19
8
Applications of LIDAR ............................................................................................................ 20
9
Conclusion: ............................................................................................................................ 25
10
Acknowledgments ............................................................................................................. 26
11
References ......................................................................................................................... 27
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LIDAR
A.
[4TH YEAR, SEMINAR]
List of Figures
Figure 1: GPS.................................................................................................................................... 8
Figure 2 : IMU .................................................................................................................................. 8
Figure 3: Distance traveled by the laser pulse (2) ........................................................................... 9
Figure 4: static LIDAR ..................................................................................................................... 10
Figure 5: a) Bathymetric LIDAR b) Topographic LIDAR (2) ...................................................... 12
Figure 6: pulse of LIDAR travles through the leaves of tree .......................................................... 13
Figure 7: One return, this is not what really happens !! ............................................................... 14
Figure 8: Multiple returns happen whenever the pulse travels through things. .......................... 14
Figure 9: Electromagnetic absorption by water (3)....................................................................... 15
Figure 10: Bathymetric LIDAR acquisition geometry (4) ............................................................. 18
Figure 11: different geologic layers in a cliff (4) ............................................................................ 20
Figure 12: damaged roof after a tornado .................................................................................... 21
Figure 13: damage to concrete in a tunnel (1) ............................................................................. 21
Figure 14: shorelines migrate over time ....................................................................................... 22
Figure 15: the intersection of the tidal datum with land .............................................................. 23
Figure 16: Wetland habitats along elevation (1) ........................................................................... 24
2
LIDAR
B.
LIDAR
GPS
IMU
[4TH YEAR, SEMINAR]
List of Abbreviation
Light Detection and Ranging
Global Positioning System
Inertial Measurement Unit
3
LIDAR
C.
[4TH YEAR, SEMINAR]
Key Words
LIDAR, Survey, Metadata, Bathymetric, Topographic.
4
LIDAR
D.
[4TH YEAR, SEMINAR]
Abstract
LIDAR data have been proven beneficial in the last few years, since it provides information like
the height and properties of objects, statistics for wide areas, and it all becomes available by
recording the intensity of backscattered pulse in addition to the 3D coordinates.
Using the different types of LIDAR help us to discover places we could not see before, like top of
mountains, see floors and other places in our wide world, depending on airborne system or
bathymetric system.
In this paper, we provide an overview of LIDAR components and basic principles to measure
things, then we review the types of LIDAR system and the advantage of each type, next we
explain how LIDAR measure things. Also, we make a list of the effective parameters which cause
errors while collecting data, and how to minimize the error of estimated values by processing
many correction steps. Finally, we mention some uses of LIDAR, and present the application of
this technology.
5
LIDAR
[4TH YEAR, SEMINAR]
2 Introduction:
Since the beginning of life on Earth, Man was obsessed discovering the world.
The discoveries helped our humanity to develop. Some people consider the discovery of
America the bonfire of our new era.
Getting more information about our world has always been difficult, and it`s getting more
difficult now days as long as our need -for more high accuracy details about our rapidly
expanding universe- is growing.
Previously, Man depended on his eyes to discover the surrounding environment, then he
worked to develop his ways depending on his abilities with the help of the new technology.
While everything has become faster, in our way to save time and reduce distances, it becomes
necessary to get the changing details of our world in a faster way. Here comes the importance of
a new technology that provides us with greater accuracy, precision, speed, and flexibility than
ever before.
“LIght Detection And Ranging (LIDAR) mapping is an accepted method of generating precise and
directly georeferenced spatial information about the shape and surface characteristics of the
Earth. Recent advancements in LIDAR mapping systems and their enabling technologies allow
scientists and mapping professionals to examine natural and built environments using a wide
range of scales.” (1)
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LIDAR
[4TH YEAR, SEMINAR]
3 Entrance for LIDAR
3.1 What is LIDAR?
LIDAR uses lasers to measure the elevations of things, like ground, forests and even buildings.
It looks like a radar which uses radio waves to map things, or sonar which uses sound waves to
map things, but a LIDAR system uses light sent out from a laser.
LIDAR, which is commonly spelled LIDAR and also known as LADAR, refers to optical remote
sensing technology which measures properties of scattered light to find range and/or other
information of a distant target.
3.2 LIDAR Component
LIDAR consists of the following separate components that are basically operating independently:





Laser ranging device.
GPS: Global Positioning System, to track plane (x,y,z) position. See figure (1).
IMU: Inertial Measurement Unit, to track planner position. See figure (2).
Inertial System, such as a computer to record data.
Digital Camera (Optional).
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LIDAR
[4TH YEAR, SEMINAR]
Figure 1: GPS
Figure 2 : IMU
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LIDAR
[4TH YEAR, SEMINAR]
3.3 The basic principle of LIDAR
The main principle of this technology is to emit a pulse of light, and then receive the returned
one in order to compute the difference in time between these two pulses.
This difference in time, with the angle at which the pulse of light was fired, and the location of
the system itself, the system will be able to give the three-dimensional coordinates of the target
object.
As long as the pulses of lasers travel at the speed of light so the distance that the pulse traveled
can be calculated using the following equation:
Distance =
And it`s divided by two since the light travels to the ground and back to the system.
Figure 3: Distance traveled by the laser pulse (2)
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LIDAR
[4TH YEAR, SEMINAR]
4 Types of LIDAR Systems
LIDAR systems can be classified into three categories:
4.1 LIDAR platforms
There`s two main methods to collect data:
-
The flown system (Airborne), where an airplane will be provided with a LIDAR system.
The ground-based stationary (Terrestrial LIDAR), where the system will be fixed to the
ground. (see figure 2)
To survey large areas, we use “flown” LIDAR, where the data is collected from planes. For
smaller areas, or where higher density is needed, it is better to use helicopters, ground-based or
water-based stationary and mobile platforms.
In order to frequently get the data about the target object, some LIDAR sensors could be
mounted on fixed-position tripods and then direct them towards targets such as bridges. This
process provides high density information.
It is obvious that the stationary systems have more accuracy than the airborne systems, but
we must use the airborne systems when we want to scan and survey large areas, because
airborne systems save time and effort.
Figure 4: static LIDAR
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LIDAR
[4TH YEAR, SEMINAR]
4.2 The Source of the Energy
Depending on the source of energy used to detect the target object, there are two types of
remote sensing technologies: passive system and active system.
Passive system detects the objects depending on the radiations that are generated by the object
itself, like the near field communication tags.
Active system does not depend on an external source. It generates the energy, then directs it
towards the target object, then detects the radiation caused by its emitted energy.
It seems it is better to depend on a passive system, since such a system will decrease consumed
power.
As long as LIDAR systems send a pulse of light and detect the reflected beam, so they are active
systems!! Actually this characteristic gives this system more advantages, since it means that
measuring and mapping could be processed anytime.
If it was a passive system, the work time would be limited to daytime only, since the radiation of
the objects will be neglected without the sun rays. But it is better to collect the data at the night
when the air is usually clearer and the sky contains less air traffic than in the daytime. This is
why it is advisable to use an active system.
4.3 The Target Object
LIDAR detects the object whatever it is; all it needs is that object radiates when the light hits it.
But if you want to detect the seafloor, you need a LIDAR that can send pulses through water.
The water penetrating capabilities of bathymetric LIDAR enable the intensity returns to be used
in detecting various seafloor features. For example, benthic habitat types can be classified using
the relative reflectance values of the returns.
The Topographic LIDAR is usually used in land surveying, urban planning, and in landscape
ecology.
Bathymetric LIDAR is a type of airborne systems that is water penetrating. Most bathymetric
LIDAR systems collect elevation and water depth simultaneously, which provides an airborne
LIDAR survey of the land-water interface. With a bathymetric LIDAR survey, the infrared light
(traditional laser system) is reflected back to the aircraft from the land and water surface, while
the additional green laser travels through the water column. Analyses of the two distinct pulses
are used to establish water depths and shoreline elevations. Bathymetric information is very
important near coastlines, in harbors, and near shores and banks. Bathymetric information is
also used to locate objects on the ocean floor.
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LIDAR
[4TH YEAR, SEMINAR]
Figure 5: a) Bathymetric LIDAR
12
b) Topographic LIDAR (2)
LIDAR
[4TH YEAR, SEMINAR]
5 How the LIDAR identifies the different objects
In this section, we will present the properties that make LIDAR capable of providing the user
with high quality information.
During the surveying process, the LIDAR system records:
1) The angle at which the laser beam was fired.
2) The difference in time between the emitted pulse and the returned one.
3) The Intensity, which refers to the amplitude of the return signal, which is a very important
parameter.
Theoretically, materials have different spectral reflectance properties resulting in different
backscattering laser intensities. Therefore, the LIDAR intensity can be used as a means to classify
and detect different materials in scans of natural or urban environments
Intensity data enables the separation of typical land, covered surfaces such as asphalt roads,
grass, trees, and house roof. Trees must be measured to determine how much wood is present,
when it is most appropriate time to harvest, and how much to harvest.
Multiple return feature allows the LIDAR technology to increase the ability to look at the threedimensional structure, since the system can capture up to five returns per pulse.
That’s because the pulse of light doesn`t just reflect and return from the top of tree only, but it
penetrates things. (see figure 8)!!
Sometimes LIDAR pulse travel through things like the gap between tree leaves. Whenever you
are setting down under a tree and you can see the sky, so the pulse of LIDAR system can reach
the ground under that tree (figure 7).
Figure 6: pulse of LIDAR travles through the leaves of tree
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LIDAR
[4TH YEAR, SEMINAR]
Figure 7: One return, this is not what really happens !!
Figure 8: Multiple returns happen whenever the pulse travels through things.
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LIDAR
[4TH YEAR, SEMINAR]
But, what about surveying seafloor? Could the LIDAR system detect the ground through water?
The bathymetric system sensors operate in the green band to penetrate water and detect
bottom features, while Many LIDAR systems operate in the near-infrared region of the
electromagnetic spectrum. It provides depth or “bathy” information out to about 70 meters in
clear water. Figure 10 shows the change in the electromagnetic absorption due to the
wavelength. (2)
Figure 9: Electromagnetic absorption by water (3)
The absorption in the green band (~532 nm) is less than when the wavelength is ~ 1064 nm.
This is why we depend on green band in the bathymetric LIDAR, while the other types of LIDAR
system works using ~1064 nm wavelength.
Combined topographic and bathymetric LIDAR systems on airborne platforms are used to map
shoreline and near shore areas.
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LIDAR
[4TH YEAR, SEMINAR]
6 Effective Parameters Influencing Intensity Measurements
Several factors influence LIDAR intensity values that can distort its ability to directly measure
reflectance.
The power of backscattered signal equation describes these effects:
Where:
*backscatter cross section: related to the incidence angle.
These factors can be divided into four main categories:
A.
Environmental Effects
Most LIDAR systems apply laser in the visible light range, which means the LIDAR systems will be
affected by dust, vapor, fog, clouds and the atmospheric effects (humidity, temperature, and
pressure), except the bathymetric LIDAR, where the system can handle the water problem.
Also, wet surfaces absorb more energy, and the reflected pulse will be weaker.
B.
Target surface characteristics
The reflection of the emitted pulse will be greater if the target`s surface has a higher reflectance
factor, because a more reflective surface will return more energy from the pulse.
Another problem may face the LIDAR systems is the highly reflective surfaces, such as glass,
mirrors, even water. This kind of object causes the multipath return, so the reflected pulses do
not represent the true properties of the target object.
Since the receiver is made to have a higher sensitivity in order to detect the weaker pulses that
returned from the target objects, so the highly reflective objects that locate close to the
detector will cause saturation, and the distance to the object is underestimated.
Also, if highly reflective objects locate far from the sensor, then these objects will appear larger,
and the estimated distance is underestimated again.
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LIDAR
C.
[4TH YEAR, SEMINAR]
Instrumental Effects
The aperture size, laser wavelength, beam divergence, and emitted power can influence the
intensity measurement; so many developments could be applied on the system in order to
reduce the estimation error.
LIDAR systems may use amplifier for low reflective surface, and may employ automatic gain
control (AGC), which increases the dynamic range that can be accommodated, other system use
Brightness reducer for near distances.
Once the returned pulse is received, the pulse power is digitized and encoded. The number of
bits the power will be scaled to, determines the accuracy of the estimation.
D.
The Measurement Geometry
The distance between the sensor and the target, and the angle the pulse is emitted, greatly
influence LIDAR intensity measurement.
Larger distance means that the pulse has to pass through more atmosphere. Since the emitted
pulse energy decays as a function of range, we notice the effects of the distance.
Greater incidence angle cause less backscattered pulses to the receiver, which gives us wrong
data about the target object.
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LIDAR
[4TH YEAR, SEMINAR]
6.2 Effective factors influencing bathymetric LIDAR intensity
measurements
As a result of the big difference between the topographic and bathymetric LIDAR, there are
additional parameters should be considered while using bathymetric LIDAR.
Since the bathymetric system dedicated to work with water, it`s important to notice that the
pulse power decays exponentially with the water depth.
Aircraft altitude, refracted beam angle, effective area of receiver optics and other factors have
an effect on the return power, and because of the scattering properties of water, the pulse will
stretch.
Figure 10: Bathymetric LIDAR acquisition geometry (4)
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LIDAR
[4TH YEAR, SEMINAR]
7 Correction Methods
The variation caused by the parameters above could be reduced by applying some processing
steps.
There are four basic levels of intensity processing. Each level increases not only with respect to
the accuracy and quality of information but also in effort required

Level 0: No modification: These are the basic intensity values directly provided by the
manufacturer. They are typically scaled to values of 0–1 (floating point), 0–255 (8-bit
integer), or 0–65,535 (16-bit integer), depending on the manufacturer.

Level 1: Intensity correction: In this process an adjustment is made to the intensity
values to reduce or ideally eliminate variation caused by one or more effective
parameters (e.g., range, angle of incidence, etc.).

Level 2: Intensity normalization: In this process an intensity image is normalized
through scaling to adjust the contrast and/or a shift to adjust the overall “brightness” to
improve matching with a neighboring tile or overlapping strip.

Level 3: Rigorous radiometric correction and calibration: In this process, the intensity
values from the LIDAR system are first evaluated on targets with known reflectance,
resulting in the determination of calibration constants for the sensor. The calibration
constants are then applied to future data that are collected with the system including
additional Level 1 intensity corrections to account for any deviations in parameters (e.g.,
range, angle of incidence). When completed rigorously, this process results in “true”
reflectance information. Hence, when radiometric calibration has been applied,
consistent data can be obtained from different systems, operated with different
parameters settings, and in different conditions.
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[4TH YEAR, SEMINAR]
8 Applications of LIDAR
As mentioned previously, the main idea is to measure the difference in time between the
emitted pulse and the returned one, record the laser angle, and then we can locate the target.
At the beginning of this technology, the main goal was to detect the ground of the target area,
and get the elevation .The first use of LIDAR data was to produce beautiful high resolution
elevation maps to detect the holes, picks, hills, even streams and roads in places we have never
discovered before.
For a long time, the trees were considered as a source of noise in the survey process, and people
worked hard to remove as much noise as possible to get high resolution elevation data.
Later, they realized that the noise -which they were trying to remove-, has very important
information, and could be very useful!!
Since that time, LIDAR has been used to get the height of the trees and buildings, tell us if it is
the right time for harvesting, and it was able to classify natural and urban cover surfaces.
In addition to that, it is used as a supplement to sensing natural environments data:
-
Identification of different rock and soil layers (Figure 4).
Snow cover change detection.
Flood modeling and wetland hydrology.
Lava flows aging.
Costal land cover mapping.
Tree classification, snag detection.
Figure 11: different geologic layers in a cliff (4)
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LIDAR
[4TH YEAR, SEMINAR]
In another field, this technology helps to detect the damage that happened to buildings or
structures. It can be used in the assessment of historic buildings, Crack detection of concrete
structures, Detection of bridge surface degradation, Detection of wind-induced cladding
damage.
(See figures 5, 6)
Figure 12: damaged roof after a tornado
Figure 13: damage to concrete in a tunnel (1)
21
LIDAR
[4TH YEAR, SEMINAR]
Talking about bathymetric LIDAR; it has been very useful to analyze the changes in shorelines
caused by the erosion, accretion and deposition. It is used to measure the parameters over time
(Figure 11).
Figure 14: shorelines migrate over time
22
LIDAR
[4TH YEAR, SEMINAR]
Also, it helps to avoid environmental disasters and make the right decision before it is too late.
Tides affect the shorelines, and it is important to keep the eyes open on tides changes, and
determine land boundary by the intersection of the tidal datum with land. (Figure 12)
Figure 15: the intersection of the tidal datum with land
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LIDAR
[4TH YEAR, SEMINAR]
As long as the wet surfaces absorb more energy from the pulse, this leads to a weaker return,
and this helps in Wetland Habitat Delineation (Figure 13).
Wetland Habitat Delineation is important in order to define the marsh habitats, species, and
their relative elevation zones, which is essential in land survey.
Figure 16: Wetland habitats along elevation (1)
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LIDAR
[4TH YEAR, SEMINAR]
9 Conclusion:
With the evolution of LIDAR systems it continues to become more useful in many applications,
and the whole world will depend basically on the different LIDAR systems to discover, collect
and update the data of different locations. A huge work has been done in order to have a high
accuracy data, by developing the correction and calibration of the collected data.
Even with that amount of research has been conducted on applying the right steps to minimize
the error of estimated values, no method can be considered as the most effective on the
resulting data, and many other correction steps can be done by developing the components or
even by applying software calibration.
For future studies, we propose to study the influence of the laser wavelength on the accuracy of
the measurement process, because the reflectance may happen only with a certain wavelength.
Another recommendation is to by attention on the scan method which can be applied using
different mechanism.
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[4TH YEAR, SEMINAR]
10 Acknowledgments
I wish to thank Dr.Nizar Zarka for his advice and comments that greatly improved the
manuscript.
I would also like to show my gratitude to Mrs. Ihsan Alhassani for sharing her pearls of wisdom
with us during the course of this paper.
I am also immensely grateful to Mr. Fahmi Alammareen for his comments during this course and
the preparation period, and for checking the grammars of this version of work.
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[4TH YEAR, SEMINAR]
11 References
1. An Introduction to Lidar Technology,Data, and Applications. [Online]
https://coast.noaa.gov/digitalcoast/_/pdf/lidar101.pdf. 4.
2. wikiwand. [Online] http://www.wikiwand.com/en/Electromagnetic_absorption_by_water. 4.
3. water absorption. [Online] http://physics.bsu.by/sites/all/other/astronomy/3-1astrophysics.html.
4. ResearchGate . [Online] https://www.researchgate.net/figure/283516821_fig4_Figure-5Bathymetric-LIDAR-acquisition-geometry-adapted-from-74. 5.
5. How a Light Detection and Ranging (LiDAR) System works. neoninc. [Online] 12 10, 2014.
[Cited: 1 24, 2015.] http://www.neoninc.org/updates-events/update/how-light-detection-andranging-lidar-system-works-%E2%80%93-newest-neon-video.
6. opentopography. Lidar Datasets from Virginia, California, Montana, Illinois, Oregon and Utah
Available. [Online] http://www.opentopography.org/news/lidar-datasets-virginia-californiamontana-illinois-oregon-and-utah-available. 3.
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