What is LIDAR?
LiDAR (Light Detection and Ranging) 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.
LIDAR systems allow scientists and
mapping professionals to examine both natural and manmade environments with accuracy, precision, and flexibility. NOAA
scientists are using LIDAR to produce more accurate shoreline maps, make
digital elevation models for use in geographic information systems, to assist
in emergency response operations, and in many other applications.
Currently we are using two types of LiDAR - Ariborne LiDAR and Ground-Based LiDAR.
Currently we are using two types of LiDAR - Ariborne LiDAR and Ground-Based LiDAR.
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. 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 know 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 totalling several terabytes.
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.
A brief history of
LiDAR
Find
out who discovered LiDAR and how?
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.
How does LiDAR work?
The science behind the technology.
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 kilometres per second, 186,000 miles per second or
0.3 metres per nanosecond so turning a light on appears to be instantaneous. Of
course, it's not! 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.
Most LiDAR systems use
four main components:
1. 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.
2. 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.
3. 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.
4. 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.
The uses of LiDAR
What
applications are there for LiDAR systems?
Airborne LiDAR Mapping
Forestry Management
and Planning
LiDAR is unique in
its ability to measure the vertical structure of forest canopies. As well as
mapping the ground beneath the forest, LiDAR is able to predict canopy bulk
density and canopy base height. Both of these factors can be used for, amongst
other things, canopy fuel capacity for use in fire behaviour models. LiDAR
surveys allow large scale surveys to be taken with a level of
cost-effectiveness not previously available. Another use of LiDAR is the
measurement of peak height to estimate the root expanse. This is a valuable
tool for insurers when considering house in particular areas
Flood Modelling
Features
like buildings, constructed river banks or roads have a great effect on flow
dynamics and flood propagation. Only high-resolution input data can solve the
purpose that relates to the systems topography as well as to the identified
features. Frequent urban flooding is observed in many parts of the world over
the past decades and an urgent need is identified to improve and increase our
modelling efforts to address the effect model input data has on the simulation
results. Even differences of a few meters can means a lot in loss calculations
in urban areas. LiDAR has brought this level of detail to the industry allowing
for much more accurate flood prediction models to be created.
Pollution
Modelling
LiDAR has a unique
ability to detect particles in both water and air. As LiDAR uses short
wavelengths of light in the visible spectrum , typically ultraviolet, visible
or near infrared, is it possible to image an object or feature only about the
same size as the wavelength or larger. This makes it particularly sensitive to
aerosols, cloud particles and air molecules. Pollutants such as carbon dioxide,
sulphur dioxide and methane are all detectable with LiDAR. Combined with a
building or terrain model this allows researchers to monitor and effectively
reduce pollutant build up is certain areas.
Urban
Planning
Urban, city, or town
planning is the discipline of land use planning which explores several aspects
of the built and social environments of municipalities and communities. LiDAR
data is a relatively new technology for obtaining Digital Surface Models (DSMs)
of the earth's surface. This data, when combined with digital orthophotos, can
be used to create highly detailed DSMs and eventually Digital City Models.
Using special software it is also possible to create estimated surface models
of buildings from the original LiDAR data. This technology allows large area
models to be created in a very short space of time.
Transport
Planning
Transportation
corridor mapping to support engineering planning and change detection of road
networks requires high spatial resolution and high scale engineering mapping
accuracy. With the latest developments of LiDAR sensors the accuracy potential
of LiDAR data has improved significantly.
Airborne
LiDAR data can be used to capture large amounts of data over large areas and
ground based LiDAR can be used to add a greater amount of details in specific
areas. This method allows the most cost effective process for site-specific
LiDAR capture.
Cellular Network
Planning
With
the ability to collect large areas of high-resolution data in a relatively
short space of time, LiDAR provides the perfect data for cellular network
planning. The detailed information can be incorporated into statistical or GIS
software and used to provide accurate analysis for determining line of sight
and view shed for proposed cellular antenna. This analysis has the benefit of
creating the optimal site for the masts ensuring coverage is optimal whilst
reducing costs in the process.
Ground-based LiDAR
Mapping
Scene of
Accident/Crime
Because
of its real-world application, LiDAR systems make recording the scene of
accidents and crime quick and easy, as well as precise. By using a ground based
LiDAR system it is possible to record the scene a car accident within a few
minutes, enabling the emergency services to clear the scene and then to
reproduce it later on in the digital realm. This reduces traffic jams as well
as preserving the evidence before anything is compromised. All of the data is
recorded with a geographical position that allows the data to be used in
various software packages for an extra level of accuracy.
Architecture
LiDAR
is a useful tool when designing and constructing new buildings. A ground based
LiDAR survey can be undertaken to give a precise digital representation of the
surrounding area and buildings. The ground can also be represented from a
ground based scan but the sensor should be moved to different locations to
ensure all details are captured.
Building
Restoration
Using
a ground based LiDAR survey it is possible to capture minute details in
building facades. This detail is a valuable record of the current condition of
a building and can be used as the basis for a digital restoration before any
work takes place. The three-dimensional data can also be printed out using the
latest in 3D Printing technology to provide a exact scale model of the
property.
Navigation
LiDAR
is becoming more and more popular as a guidance system for autonomous vehicles.
The speed and accuracy of a scanner means that data can be passed to a system
to process the return in more or less real-time. This allows the device
controlling the vehicle to detect obstacles and to update its route in a very
small amount of time.
Military
and law enforcement
One
situation where LiDAR has notable non-scientific application is in traffic
speed law enforcement, for vehicle speed measurement, as a technology
alternative to radar guns. The technology for this application is small enough
to be mounted in a hand held camera "gun" and permits a particular
vehicle's speed to be determined from a stream of traffic.
Unlike
RADAR which relies on Doppler shifts to directly measure speed, police lidar
relies on the principle of time-of-flight to calculate speed. The equivalent
radar based systems are often not able to isolate particular vehicles from the
traffic stream and are generally too large to be hand held.
Driverless Car / Smart Car
The
Google Self-Driving Car is a project by Google that involves developing
technology for autonomous cars, mainly electric cars. The software powering
Google's cars is called Google Chauffeur.
Google's
robotic cars have about $150,000 in equipment including a $70,000 LIDAR system.
The range finder mounted on the top is a Velodyne 64-beam laser. This laser
allows the vehicle to generate a detailed 3D map of its environment. The car
then takes these generated maps and combines them with high-resolution maps of
the world, producing different types of data models that allow it to drive
itself.
There are also some Advantages and some Disadvantages of LiDAR -
Advantages of LiDAR
- All data geo-referenced from
inception
- High level of accuracy
- Ability to cover large areas
quickly
- Quicker turnaround, less labor
intensive, and lower costs than photogrammetric methods
- Can collect data in steep terrain
and shadows
- Can produce DEM and DSM
Disadvantages of LiDAR
- Inability to penetrate very dense
canopy leads to elevation model errors
- Very large datasets that are
difficult to interpret and process
- No international protocols
- High Cost
Intresting
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