Basic introduction to LiDAR

1. Compiled by: Atiqa Ijaz Khan
Friday, 09 December, 2016|
INTRODUCTION TO
LiDAR
For Guide and Understanding

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TABLE OF CONTENTS
What is Light Detection and Ranging (LiDAR)?..................................................................3
Major Components of LiDAR ................................................................................................3
Sculpt laser-accurate outputs using LiDAR..........................................................................3
As an Optical Sensor..............................................................................................................4
1. Number of Returns..........................................................................................................5
Complete Cycle of Return Beams......................................................................................6
2. Digital Elevation Models ................................................................................................7
3. Canopy Height Model (CHM)........................................................................................8
For Example:......................................................................................................................8
4. Light Intensity.................................................................................................................8
For Example.......................................................................................................................9
5. Point Classification.........................................................................................................9
Point Cloud ......................................................................................................................11
LiDAR data is a rare, precious GIS resource......................................................................12
From ground to air, explore the types of LiDAR systems..................................................13
1. Profiling LiDAR ...........................................................................................................13
2. Small Footprint LiDAR ................................................................................................13
Two types of LIDAR are topographic and bathymetric: .....................................................13
i. Topographic LIDAR.....................................................................................................13
ii. Bathymetric LiDAR.....................................................................................................13
3. Large Footprint LiDAR ................................................................................................14
4. Ground-based LiDAR...................................................................................................14
You have x-ray vision using these LiDAR applications .....................................................14
1. Riparian ecologists........................................................................................................14
2. Foresters........................................................................................................................14

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3. Google’s self-driving ....................................................................................................15
4. Archaeologists...............................................................................................................15
LiDAR system components: breaking it down....................................................................15
Storage of the return: full waveform vs discrete LiDAR ...................................................15
Light Detection and Ranging is moving towards a full waveform system..........................16
Conclusion ..............................................................................................................................16
Courtesy To:...........................................................................................................................17

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INTRODUCTION TO LIDAR
WHAT IS LIGHT DETECTION AND RANGING (LIDAR)?
How would you like to wave your magic wand and all of a sudden find out how far everything
is away from you? No magic wands necessary. This is how LiDAR (Light Detection and
Ranging) works – minus the magic wand. LiDAR is fundamentally a distance technology. An
airborne LiDAR system actively sends light energy to the ground. This light emitted is known
as a pulse. The LiDAR measures reflected light back to the sensor. This reflected light is
known as a return. So, pulses of light travel to the ground. They return and are detected by
the sensor giving the range (a variable distance) to the Earth. This is how LiDAR earned its
name – Light Detection and Ranging. That was easy.
Lidar (light detection and ranging) is an optical remote-sensing technique that uses laser light
to densely sample the surface of the earth, producing highly accurate x,y,z measurements.
Lidar, primarily used in airborne laser mapping applications, is emerging as a cost-effective
alternative to traditional surveying techniques such as photogrammetry. Lidar produces mass
point cloud datasets that can be managed, visualized, analyzed, and shared.
MAJOR COMPONENTS OF LIDAR
The major hardware components of a lidar system include a collection vehicle (aircraft,
helicopter, vehicle, and tripod), laser scanner system, GPS (Global Positioning System), and
INS (inertial navigation system). An INS system measures roll, pitch, and heading of the lidar
system.
Need LiDAR data? You’ll want to see our top 6 free LiDAR data sources.
SCULPT LASER-ACCURATE OUTPUTS USING LIDAR
Light detection and ranging is active remote sensing. This means the LiDAR itself sends a
pulse of near infrared light and it waits for the pulse to return. This is sensors which collects
reflected energy from the sun. Active sensors are very accurate because it’s being controlled in
the platform.

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LiDAR is a sampling tool. It has the brute force to send 160,000 pulses per second. It creates
millions of points. Point density is usually less than one meter with accuracy of about 15 cm
vertically and 40 cm horizontally.
Figure 1. Airborne Light Detection and Ranging (LiDAR)
A LiDAR unit scans the ground from side to side as the plane flies because this covers a larger
area. Some pulses will be directly at nadir. But most pulses travel at an angle (off-nadir). So, a
LiDAR system accounts for angle when it calculates elevation.
AS AN OPTICAL SENSOR
Lidar is an active optical sensor that transmits laser beams toward a target while moving
through specific survey routes. The reflection of the laser from the target is detected and
analyzed by receivers in the lidar sensor. These receivers record the precise time from when
the laser pulse left the system to when it is returned to calculate the range distance between the
sensor and the target. Combined with the positional information (GPS and INS), these distance
measurements are transformed to measurements of actual three-dimensional points of the
reflective target in object space. The point data is post-processed after the lidar data collection
survey into highly accurate georeferenced x,y,z coordinates by analyzing the laser time range,
laser scan angle, GPS position, and INS information.
Figure 2. LiDAR Beam

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1. NUMBER OF RETURNS
Imagine you’re hiking in a forest. You look up.
Figure 3. Sunlight through Forest Canopy
If you can see light, this means that LiDAR pulses can go through too. This means that LiDAR
can also hit the bare Earth or short vegetation. A significant amount of the LIDAR energy can
penetrate the forest canopy just like sunlight.
But LiDAR won’t necessarily only hit the bare ground. In a forested area, it can reflect off
different parts of the forest until the pulse finally hits the ground:
Using a LiDAR to get bare ground points, you’re not x-raying through vegetation. You’re
really peering through the gaps in the leaves. LiDAR collects a massive number of points.
These multiple hits of the branches are the number of returns.
Figure 4. Number of Returns
In a forest, the laser pulse goes down. We get reflections from different parts of the forest –
1st, 2nd, 3rd returns until it finally hits the bare ground. If there’s no forest in the way, it will
just hit the surface.

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Sometimes a pulse of light doesn’t reflect off one thing. As with the case of trees, one light
pulse could have multiple returns. LiDAR systems can record information starting from the top
of the canopy through the canopy all the way to the ground. This makes LiDAR highly valuable
for understanding forest structure and shape of the trees.
aser pulses emitted from a lidar system reflect from objects both on and above the ground
surface: vegetation, buildings, bridges, and so on. One emitted laser pulse can return to the
lidar sensor as one or many returns. Any emitted laser pulse that encounters multiple reflection
surfaces as it travels toward the ground is split into as many returns as there are reflective
surfaces.
COMPLETE CYCLE OF RETURN BEAMS
The first returned laser pulse is the most significant return and will be associated with the
highest feature in the landscape like a treetop or the top of a building. The first return can also
represent the ground, in which case only one return will be detected by the lidar system.
Multiple returns are capable of detecting the elevations of several objects within the laser
footprint of an outgoing laser pulse. The intermediate returns, in general, are used for
vegetation structure, and the last return for bare-earth terrain models.
The last return will not always be from a ground return. For example, consider a case where a
pulse hits a thick branch on its way to the ground and the pulse does not actually reach the
ground. In this case, the last return is not from the ground but from the branch that reflected
the entire laser pulse.
Figure 5. LiDAR Laser Return

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2. DIGITAL ELEVATION MODELS
How do you build a Digital Elevation Model from LiDAR? Digital Elevation Models are bare
earth (topology) models of the Earth’s surface. You can derive Digital Elevation Models (or
Digital Terrain Models) by using the ground hits from LiDAR. Ground hits are the last return
of the LiDAR.
Figure 6. Digital Elevation Model (DEM)
Sometimes the last return may not even make it to the bare ground. But for LiDAR, this is rarer
than you think. Which points are ground hits? There are ways to filter the LiDAR points. Take
the ground hits (topology only) meaning the last returns from LiDAR. Filter last return points.
Interpolate.
Build your DEM. With a DEM, you can generate products like slope (rise or fall expressed in
degrees or percent), aspect (slope direction) and hill shade (shaded relief considering
illumination angle) maps.
Figure 7. LiDAR Digital Elevation Model

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3. CANOPY HEIGHT MODEL (CHM)
Light detection and ranging attains very accurate information about the ground surface. We
can also get very accurate information about what’s on top of the ground. A Canopy Height
Models (Normalized Digital Surface Model (nDSM)) gives you true height of topological
features on the ground. So how do you get true height of features on the Earth? Take the first
return including topology (tree, building). Subtract the last return which are the ground hits
(bare Earth).
Figure 8. Normalized Digital Surface Model
FOR EXAMPLE:
The top of the tree height minus the ground height. Interpolate the result. You get a surface of
features real height on the ground.
Figure 9. LiDAR Canopy Height Model
4. LIGHT INTENSITY
The strength of LiDAR returns varies with the composition of the surface object reflecting the
return. The reflective percentages are referred to as LiDAR intensity.

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Figure 10. Light Intensity
But a number of factors affect light intensity. Range, incident angle, beam, receiver and surface
composition (especially) influences light intensity. When the pulse is tilted further away, the
return energy decreases. Light intensity is particularly useful in distinguishing features in land
use/cover.
FOR EXAMPLE
Impervious surfaces stand out in light intensity images. Object-based image classification
segmentation can separate these features using light intensity values.
Figure 11. Impervious Surfaces
5. POINT CLASSIFICATION
Additional information is stored along with every x, y, and z positional value. The following
lidar point attributes are maintained for each laser pulse recorded: intensity, return number,
number of returns, point classification values, points that are at the edge of the flight line, RGB
(red, green, and blue) values, GPS time, scan angle, and scan direction. The following table
describes the attributes that can be provided with each lidar point. LiDAR data sets may already
be classified by the vendor with a point classification. The codes are generated by the reflected

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laser pulse in a semi-automatic way. Not all vendors add this LAS classification field. (It is
usually agreed in the contract beforehand).
Figure 12. Point Classification
The American Society for Photogrammetry and Remote Sensing (ASPRS) has defined a list of
classification codes for LiDAR. Classes include ground, vegetation (low, medium and high),
building, water, unassigned, etc. Point classification may fall into more than one category.
These points are usually flagged and have secondary classes.
NO. LIDAR ATTRIBUTE DESCRIPTION
1 Intensity The return strength of the laser pulse that generated the lidar
point.
2 Return number An emitted laser pulse can have up to five returns depending on
the features it is reflected from and the capabilities of the laser
scanner used to collect the data. The first return will be flagged as
return number one, the second as return number two, and so on.
3 Number of returns The number of returns is the total number of returns for a given
pulse. For example, a laser data point may be return two (return
number) within a total number of five returns.
4 Point classification Every lidar point that is post-processed can have a classification
that defines the type of object that has reflected the laser pulse.
Lidar points can be classified into a number of categories

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including bare earth or ground, top of canopy, and water. The
different classes are defined using numeric integer codes in the
LAS files.
5 Edge of flight line The points will be symbolized based on a value of 0 or 1. Points
flagged at the edge of the flight line will be given a value of 1,
and all other points will be given a value of 0.
7 RGB Lidar data can be attributed with RGB (red, green, and blue)
bands. This attribution often comes from imagery collected at the
same time as the lidar survey.
9 GPS time The GPS time stamp at which the laser point was emitted from
the aircraft. The time is in GPS seconds of the week.
10 Scan angle The scan angle is a value in degrees between -90 and +90. At 0
degrees, the laser pulse is directly below the aircraft at nadir. At -
90 degrees, the laser pulse is to the left side of the aircraft, while
at +90, the laser pulse is to the right side of the aircraft in the
direction of flight. Most lidar systems are currently less than ±30
degrees.
11 Scan direction The scan direction is the direction the laser scanning mirror was
traveling at the time of the output laser pulse. A value of 1 is a
positive scan direction, and a value of 0 is a negative scan
direction. A positive value indicates the scanner is moving from
the left side to the right side of the in-track flight direction, and a
negative value is the opposite.
POINT CLOUD
Post-processed spatially organized lidar data is known as point cloud data. The initial point
clouds are large collections of 3D elevation points, which include x, y, and z, along with
additional attributes such as GPS time stamps. The specific surface features that the laser
encounters are classified after the initial lidar point cloud is post-processed. Elevations for the
ground, buildings, forest canopy, highway overpasses, and anything else that the laser beam
encounters during the survey constitutes point cloud data. Lidar point cloud data in some

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software like, ArcGIS is most commonly represented as either a series of rasters or TINs, a
LAS dataset, a terrain dataset, or a mosaic dataset.
Figure 13. LiDAR Point Cloud Classification
For more information on LiDAR point classification:
 http://desktop.arcgis.com/en/arcmap/10.3/manage-data/las-dataset/lidar-point-
classification.htm
 http://desktop.arcgis.com/en/arcmap/10.3/manage-data/las-dataset/working-with-las-
classifications-in-arcgis.htm
LIDAR DATA IS A RARE, PRECIOUS GIS RESOURCE
Light detection and ranging is accurate, large-scale and covers the most ground. You can
understand bare ground elevation, canopy heights, light intensity and more. Anyone who is
serious about understanding landscape topology should use LiDAR.
Figure 14. Landscape Topology

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But LiDAR is a beast of a data set to work with. LiDAR is stored in LAS file format as a point
cloud. This file format is maintained by ASPRS. The LAS format facilitates exchange between
vendors and customers with no information being lost.
Figure 15. Nothing is better than free
But in most cases, you will have to purchase LiDAR data. LiDAR is generally flown
commercially by helicopter, airplane and drone.
FROM GROUND TO AIR, EXPLORE THE TYPES OF LIDAR SYSTEMS
1. PROFILING LIDAR
Was the first type of Light Detection and Ranging used in the 1980s for single line features
such as power lines. Profiling LiDAR sends out an individual pulse in one line. It measures
height along a single transect with a fixed Nadir angle.
2. SMALL FOOTPRINT LIDAR
Is what we use today. Small-footprint LiDAR scans at about 20 degrees moving backwards
and forwards (scan angle). If it goes beyond 20 degrees, the LiDAR instrument may start seeing
the sides of trees instead of straight down.
TWO TYPES OF LIDAR ARE TOPOGRAPHIC AND BATHYMETRIC:
I. TOPOGRAPHIC LIDAR
Maps the land typically using near-infrared light.
II. BATHYMETRIC LIDAR
Uses water-penetrating green light to measure seafloor and riverbed elevations.

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3. LARGE FOOTPRINT LIDAR
Uses full waveforms and averages LiDAR returns in 20 m footprints. But it’s very difficult to
get terrain from large footprint LiDAR because you get a pulse return based on a larger area
which could be sloping. There are generally less applications for large footprint LiDAR. Only
SLICER (Scanning Lidar Imager of Canopies by Echo Recovery) and LVIS (Laser Vegetation
Imaging Sensor), both built by NASA and are experimental.
4. GROUND-BASED LIDAR
Sits on a tripod and scans the hemisphere. Ground-based LiDAR is good for scanning
buildings. It’s used in geology, forestry, heritage preservation and construction applications.
YOU HAVE X-RAY VISION USING THESE LIDAR APPLICATIONS
Figure 16. LiDAR Sample
Light detection and ranging is being used every day in surveying, forestry, urban planning and
more. Here are a couple of LiDAR applications that stand out:
1. RIPARIAN ECOLOGISTS
Use LiDAR to delineate stream orders. With a LiDAR-derived DEM, tributaries become clear.
It’s easier to see where they go far superior than standard aerial photography.
2. FORESTERS
Use LiDAR to better understand forest structure and shape of the trees because one light pulse
could have multiple returns. As with the case of trees, LiDAR systems can record information
starting from the top of the canopy through the canopy all the way to the ground.

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3. GOOGLE’S SELF-DRIVING
If car got pulled over by the cops, how would it react? Self-driving cars use Light Detection
and Ranging? The first secret behind Google’s self-driving car is LiDAR scanner. It detects
pedestrians, cyclists stop signs and other obstacles.
4. ARCHAEOLOGISTS
Have used LiDAR to find subtle variations in elevation on the ground. It was a bit of a surprise
when archaeologists found square patterns on the ground over vegetation. These square
patterns were ancient buildings and pyramids by ancient Mayan and Egyptian civilizations.
LIDAR SYSTEM COMPONENTS: BREAKING IT DOWN
How does a light detection and ranging system work? There are 4 parts of an airborne LiDAR.
These 4 parts of a LiDAR system work together to produce highly accurate, usable results:
 LiDAR sensors scan the ground from side to side as the plane flies. The sensor is
commonly in green or near-infrared bands.
 GPS receivers track the altitude and location of the airplane. These variables are important
in attaining accurate terrain elevation values.
 Inertial measurement units track the tilt of the airplane as it flies. Elevation calculations
use tilt to accurately measure incident angle of the pulse.
 Computers (Data Recorders) record all of the height information as the LiDAR scans the
surface.
These LiDAR components cohesively make up a Light Detection and Ranging system.
STORAGE OF THE RETURN: FULL WAVEFORM VS DISCRETE LIDAR
Light detection and ranging return pulses are stored in two ways:
 Full waveform
 Discrete LiDAR
What are the differences between full waveform and discrete LiDAR systems? Imagine that in
the forest that LiDAR pulse is being hit by branches multiple times. Pulses are coming back as
1st, 2nd, 3rd returns. Then you get a large pulse by the bare ground return. When you record

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the data as separate returns, this is called Discrete Return LiDAR. Discrete takes each peak
and separates each return.
Figure 17. Discrete Return LiDAR
LIGHT DETECTION AND RANGING IS MOVING TOWARDS A FULL WAVEFORM SYSTEM
When you record the WHOLE RETURN as one continuous wave, this would be called full-
waveform LiDAR. Full waveform data is more complicated. You can simply count the peaks
and that makes it discrete.
Figure 18. Full Waveform
CONCLUSION
Light Detection and Ranging uses lasers to measure the elevation of features like forests,
buildings and the bare earth. It’s similar to sonar (sound waves) or radar (radio waves) because
it sends a pulse and measures the time it takes to return. But LiDAR is different than sonar and
radar because it uses light. This means LiDAR is an active remote sensing system. The
applications for LiDAR is stunning. It’s definitely growing in GIS. Forest structure,
archaeology, land use mapping, flood modelling, transportation planning, architecture, oil and
gas exploration, public safety, automated vehicles, military and conservation. If we had a nickel
for everywhere LiDAR is being integrated, we’d be Bruce Wayne rich.

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COURTESY TO:
 http://gisgeography.com/lidar-light-detection-and-ranging/
 http://gisgeography.com/top-6-free-lidar-data-sources/
 http://gisgeography.com/image-classification-techniques-remote-sensing/
 http://www.asprs.org/
 https://www.asprs.org/committee-general/laser-las-file-format-exchange-activities.html
 http://gisgeography.com/100-earth-remote-sensing-applications-uses
 http://desktop.arcgis.com/en/arcmap/10.3/manage-data/las-dataset/what-is-lidar-data-.htm