Light Detection and Ranging
ABSTRACT
LiDAR is an acronym for Light Detection And Ranging, sometimes also referred to as
Laser Altimetry or Airborne Laser Terrain Mapping (ALTM). The LiDAR system
basically consists of integration of three technologies, namely, Inertial Navigation
System (INS), LASER, and GPS. The Global Positioning System (GPS) has been
fully operational for over a decade, and during this period, the technology has proved
its potential in various application areas. Some of the important applications of GPS
are crustal deformation studies, vehicle guidance systems, and more recently, in
LiDAR.
Geo Spatial Information is an important input for all planning and developmental
activities especially in the present era of digital mapping and decision support
systems. LiDAR is much faster than conventional photogrammetric technology and
offers distinct advantage over photogrammetry in some application areas. Its
development goes back to 1970s and 1980s, with the introduction of the early NASALiDAR systems, and other attempts in USA and Canada (Ackermann, 1999). The
method has successfully established itself as an important data collection technique,
within a few years, and quickly spread into practical applications. Early 1980's,
second generation LiDAR systems were in use around the world but were expensive
and had limited capability. With the enhanced computer power available today, and
with the latest positioning and orientation systems, LiDAR systems have become a
commercially viable alternative for development of Digital Elevation Models (DEM)
of earth surface.
Light Detection and Ranging
CONTENTS
1. Introduction
2. What is LIDAR?
3. Operation
4. Design
5. Applications
6. Advantages
7. Disadvantages
8. Conclusion
9. References
Light Detection and Ranging
INTRODUCTION
Light Detection And Ranging is an optical remote sensing technology that measures
properties of scattered light to find range and/or other information of a distant target.
The prevalent method to determine distance to an object or surface is to use laser
pulses. Like the similar radar technology, which uses radio waves, the range to an
object is determined by measuring the time delay between transmission of a pulse and
detection of the reflected signal. LIDAR technology has application in Geomatics,
archaeology, geography, geology, geomorphology, seismology, forestry, remote
sensing and atmospheric physics. Applications of LIDAR include ALSM (Airborne
Laser Swath Mapping), laser altimetry or LIDAR Contour Mapping. The acronym
LADAR (Laser Detection and Ranging) is often used in military contexts. The term
"laser radar" is also in use even though LIDAR does not employ microwaves or radio
waves, which is definitional to radar.
The primary difference between LIDAR and radar is that LIDAR uses much shorter
wavelengths of the electromagnetic spectrum, typically in the ultraviolet, visible, or near
infrared range. In general it is possible to image a feature or object only about the same size
as the wavelength, or larger. Thus lidar is highly sensitive to aerosols and cloud particles and
has many applications in atmospheric research and meteorology.
An object needs to produce a dielectric discontinuity to reflect the transmitted wave.
At radar (microwave or radio) frequencies, a metallic object produces a significant
reflection. However non-metallic objects, such as rain and rocks produce weaker
reflections and some materials may produce no detectable reflection at all, meaning
some objects or features are effectively invisible at radar frequencies. This is
especially true for very small objects (such as single molecules and aerosols).
Lasers provide one solution to these problems. The beam densities and coherency are
excellent. Moreover the wavelengths are much smaller than can be achieved with
radio systems, and range from about 10 micrometers to the UV (ca. 250 nm). At such
wavelengths, the waves are "reflected" very well from small objects. This type of
reflection is called backscattering. Different types of scattering are used for different
lidar applications, most common are Rayleigh scattering , Mie scattering and Raman
Light Detection and Ranging
scattering as well as fluorescence. Based on different kinds of backscattering, the
LIDAR can be accordingly called Rayleigh LiDAR, Mie LiDAR, Raman LiDAR and
Na/Fe/K Fluorescence LIDAR and so on. The wavelengths are ideal for making
measurements of smoke and other airborne particles (aerosols), clouds, and air
molecules.
A laser typically has a very narrow beam which allows the mapping of physical
features with very high resolution compared with radar. In addition, many chemical
compounds interact more strongly at visible wavelengths than at microwaves,
resulting in a stronger image of these materials. Suitable combinations of lasers can
allow for remote mapping of atmospheric contents by looking for wavelengthdependent changes in the intensity of the returned signal.
LIDAR has been used extensively for atmospheric research and meteorology. With
the deployment of the GPS in the 1980s precision positioning of aircraft became
possible. GPS based surveying technology has made airborne surveying and mapping
applications possible and practical. Many have been developed, using downwardlooking LIDAR instruments mounted in aircraft or satellites. A recent example is the
NASA Experimental Advanced Research Lidar.
Light Detection and Ranging
What's a Lidar?
A lidar is similar to the more familiar radar, and can be thought of as a laser radar.
In a radar, radio waves are transmitted into the atmosphere, which scatters some of the
power back to the radar's receiver.
A lidar also transmits and receives electromagnetic radiation, but at a higher
frequency. Lidars operate in the ultraviolet, visible and infrared region of the
electromagnetic spectrum.
. Choosing different types of scattering processes allows atmospheric composition,
temperature and wind to be measured.
LIDAR is an acronym which stands for LIghtDetection AndRanging (radar is also an
acronym).
A simplified block diagram of a lidar contains a transmitter, receiver and detector
system. Different types of physical processes in the atmosphere are related to different
types of light scattering
The lidar's transmitter is a laser, while its receiver is an optical telescope.
Light Detection and Ranging
A FASOR used at the Starfire Optical Range for LIDAR and laser guide star
experiments is tuned to the sodium D2a line and used to excite sodium atoms in the
upper atmosphere.
This lidar (laser range finder) may be used to scan buildings, rock formations, etc., to
produce a 3D model. The lidar can aim its laser beam in a wide range: its head rotates
horizontally, a mirror flips vertically. The laser beam is used to measure the distance
to the first object on its path.
Light Detection and Ranging
OPERATION
Different kinds of lasers are used depending on the power and wavelength required.
The lasers may be both cw (continuous wave, on continuous like a light bulb) or
pulsed (like a strobe light).
Gain mediums for the lasers include, gases (e.g. HeNe = Helium Neon or Xenon
Fluoride), solid state diodes, dyes and crystals (e.g. Nd:YAG = Neodymium:Yttrium
Aluminum Garnet).
For some lidar applications more than 1 kind of laser is used. Here is the transmitter
for Western's Purple Crow Lidar, which uses both cw and pulsed lasers. The final
output of both channels of the transmitter is pulsed with a pulse repetition rate of 20
times per second and a pulse width of about 7 ns (1 ns = 1 x 10-9s) .
The receiving system records the scattered light received by the receiver at fixed time
intervals.
Lidars typically use extremely sensitive detectors called photomultiplier tubes to
detect the backscattered light.
Photomultiplier tubes (shown below) convert the individual quanta of light, photons,
first into electric currents and then into digital photocounts which can be stored and
processed on a computer.
Light Detection and Ranging
Amazing when you consider the electric currents generated are on the order of
picoamps (1 pA = 10-12 A; a 60 W light bulb draws a current of 0.5 A!).
The photocounts received are recorded for fixed time intervals during the return pulse.
The times are then converted to heights called range bins since the speed of light is
well known.
So to find range bins from time:
wherec is the speed of light, 3 x 108 m/s.
So if each range bin is 160 ns long the height of each bin is 24 m.
The range-gated photocounts are then stored and analyzed by a computer.
Sources for information in this section can be found here.
Light Detection and Ranging
DESIGN
In general there are two kinds of lidar detection schema: "incoherent" or direct energy
detection (which is principally an amplitude measurement) and Coherent detection
(which is best for doppler, or phase sensitive measurements). Coherent systems
generally use Optical heterodyne detection which being more sensitive than direct
detection allows them to operate a much lower power but at the expense of more
complex transceiver requirements.
In both coherent and incoherent LIDAR, there are two types of pulse models:
micropulselidar systems and high energy systems. Micropulse systems have
developed as a result of the ever increasing amount of computer power available
combined with advances in laser technology. They use considerably less energy in the
laser, typically on the order of one microjoule, and are often "eye-safe," meaning they
can be used without safety precautions. High-power systems are common in
atmospheric research, where they are widely used for measuring many atmospheric
parameters: the height, layering and densities of clouds, cloud particle properties
(extinction coefficient, backscatter coefficient, depolarization), temperature, pressure,
wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.)[1].
There are several major components to a LIDAR system:
1. Laser — 600-1000 nm lasers are most common for non-scientific
applications. They are inexpensive but since they can be focused and easily
absorbed by the eye the maximum power is limited by the need to make them
eye-safe. Eye-safety is often a requirement for most applications. A common
alternative 1550 nm lasers are eye-safe at much higher power levels since this
wavelength is not focused by the eye, but the detector technology is less
advanced and so these wavelengths are generally used at longer ranges and
lower accuracies. They are also used for military applications as 1550 nm is
not visible in night vision goggles unlike the shorter 1000 nm infrared laser.
Airborne topographic mapping lidars generally use 1064 nm diode pumped
YAG lasers, while bathymetric systems generally use 532 nm frequency
doubled diode pumped YAG lasers because 532 nm penetrates water with
much less attenuation than does 1064 nm. Laser settings include the laser
Light Detection and Ranging
repetition rate (which controls the data collection speed). Pulse length is
generally an attribute of the laser cavity length, the number of passes required
through the gain material (YAG, YLF, etc.), and Q-switch speed. Better target
resolution is achieved with shorter pulses, provided the LIDAR receiver
detectors and electronics have sufficient bandwidth.
2. Scanner and optics — How fast images can be developed is also affected by
the speed at which it can be scanned into the system. There are several options
to scan the azimuth and elevation, including dual oscillating plane mirrors, a
combination with a polygon mirror, a dual axis scanner. Optic choices affect
the angular resolution and range that can be detected. A hole mirror or a beam
splitter are options to collect a return signal.
3. Photodetector and receiver electronics — Two main photodetector
technologies are used in lidars: solid state photodetectors, such as silicon
avalanche photodiodes, or photomultipliers. The sensitivity of the receiver is
another parameter that has to be balanced in a LIDAR design.
4. Position and navigation systems — LIDAR sensors that are mounted on
mobile platforms such as airplanes or satellites require instrumentation to
determine the absolute position and orientation of the sensor. Such devices
generally include a Global Positioning System receiver and an Inertial
Measurement Unit (IMU).
Light Detection and Ranging
Applications
This LIDAR-equipped mobile robot uses its LIDAR to construct a map and avoid
obstacles.
Other than those applications listed above, there are a wide variety of applications of
LIDAR, as often mentioned in National LIDAR Dataset programs.
Archaeology
LIDAR has many applications in the field of archaeology including aiding in the
planning of field campaigns, mapping features beneath forest canopy[3], and providing
an overview of broad, continuous features that may be indistinguishable on the
ground. LIDAR can also provide archaeologists with the ability to create highresolution digital elevation models (DEMs) of archaeological sites that can reveal
micro-topography that are otherwise hidden by vegetation. LiDAR-derived products
can be easily integrated into a Geographic Information System (GIS) for analysis and
interpretation. For example at Fort Beausejour - Fort Cumberland National Historic
Site, Canada, previously undiscovered archaeological features have been mapped that
are related to the siege of the Fort in 1755. Features that could not be distinguished on
the ground or through aerial photography were identified by overlaying hillshades of
the DEM created with artificial illumination from various angles. With LiDAR the
ability to produce high-resolution datasets quickly and relatively cheaply can be an
advantage. Beyond efficiency, its ability to penetrate forest canopy has led to the
discovery of features that were not distinguishable through traditional geo-spatial
methods and are difficult to reach through field surveys.
Light Detection and Ranging
Meteorology and Atmospheric Environment
The first LIDAR systems were used for studies of atmospheric composition, structure,
clouds, and aerosols. Initially based on ruby lasers, LIDAR for meteorological
applications was constructed shortly after the invention of the laser and represent one
of the first applications of laser technology.
Elastic backscatter LIDAR is the simplest type of lidar and is typically used for
studies of aerosols and clouds. The backscattered wavelength is identical to the
transmitted wavelength, and the magnitude of the received signal at a given range
depends on the backscatter coefficient of scatterers at that range and the extinction
coefficients of the scatterers along the path to that range. The extinction coefficient is
typically the quantity of interest.
Differential Absorption LIDAR (DIAL) is used for range-resolved measurements of a
particular gas in the atmosphere, such as ozone, carbon dioxide, or water vapor. The
LIDAR transmits two wavelengths: an "on-line" wavelength that is absorbed by the
gas of interest and an off-line wavelength that is not absorbed. The differential
absorption between the two wavelengths is a measure of the concentration of the gas
as a function of range. DIAL LIDARs are essentially dual-wavelength elastic
backscatter LIDARS.
Raman LIDAR is also used for measuring the concentration of atmospheric gases, but
can also be used to retrieve aerosol parameters as well. Raman LIDAR exploits
inelastic scattering to single out the gas of interest from all other atmospheric
constituents. A small portion of the energy of the transmitted light is deposited in the
gas during the scattering process, which shifts the scattered light to a longer
wavelength by an amount that is unique to the species of interest. The higher the
concentration of the gas, the stronger the magnitude of the backscattered signal.
Doppler LIDAR is used to measure wind speed along the beam by measuring the
frequency shift of the backscattered light. Scanning LIDARs, such as NASA's
HARLIE LIDAR, have been used to measure atmospheric wind velocity in a large
three dimensional cone. ESA's wind mission ADM-Aeolus will be equipped with a
Doppler LIDAR system in order to provide global measurements of vertical wind
Light Detection and Ranging
profiles. A doppler LIDAR system was used in the 2008 Summer Olympics to
measure wind fields during the yacht competition.Doppler LIDAR systems are also
now beginning to be successfully applied in the renewable energy sector to acquire
wind speed, turbulence, wind veer and wind shear data. Both pulsed and continuous
wave systems are being used. Pulsed systems using signal timing to obtain vertical
distance resolution, whereas continuous wave systems rely on detector focusing.
Synthetic Array LIDAR allows imaging LIDAR without the need for an array detector.
It can be used for imaging Doppler velocimetry, ultra-fast frame rate (MHz) imaging,
as well as for speckle reduction in coherent LIDAR.
Wind power
Lidar is sometimes used on wind farms to more accurately measure wind speeds and
wind turbulence, and an experimentallidar is mounted on a wind turbine rotor to
measure oncoming horizontal winds, and proactively adjust blades to protect
components and increase power.
Geology and Soil Science
High-resolution digital elevation maps generated by airborne and stationary LIDAR
have led to significant advances in geomorphology, the branch of geoscience
concerned with the origin and evolution of Earth's surface topography. LIDAR's
abilities to detect subtle topographic features such as river terraces and river channel
banks, measure the land surface elevation beneath the vegetation canopy, better
resolve spatial derivatives of elevation, and detect elevation changes between repeat
surveys have enabled many novel studies of the physical and chemical processes that
shape landscapes. In addition to LIDAR data collected by private companies,
academic consortia have been created to support the collection, processing and
archiving of research-grade, publicly available LIDAR datasets. The National Center
for Airborne Laser Mapping (NCALM), supported by the National Science
Foundation, collects and distributes LIDAR data in support of scientific research and
education in a variety of fields, particularly geoscience and ecology.
Light Detection and Ranging
In geophysics and tectonics, a combination of aircraft-based LIDAR and GPS have
evolved into an important tool for detecting faults and measuring uplift. The output of
the two technologies can produce extremely accurate elevation models for terrain that
can even measure ground elevation through trees. This combination was used most
famously to find the location of the Seattle Fault in Washington, USA. This
combination is also being used to measure uplift at Mt. St. Helens by using data from
before and after the 2004 uplift. Airborne LIDAR systems monitor glaciers and have
the ability to detect subtle amounts of growth or decline. A satellite based system is
NASA'sICESat which includes a LIDAR system for this purpose. NASA's Airborne
Topographic Mapper is also used extensively to monitor glaciers and perform coastal
change analysis. The combination is also used by soil scientists while creating a soil
survey. The detailed terrain modeling allows soil scientists to see slope changes and
landform breaks which indicate patterns in soil spatial relationships.
Agriculture
Agricultural Research Service scientists have developed a way to incorporate LIDAR
with yield rates on agricultural fields. This technology will help farmers direct their
resources toward the high-yield sections of their land.
LIDAR also can be used to help farmers determine which areas of their fields to apply
costly fertilizer. LIDAR can create a topological map of the fields and reveals the
slopes and sun exposure of the farm land. Researchers at the Agricultural Research
Service blended this topological information with the farm land’s yield results from
previous years. From this information, researchers categorized the farm land into
high-, medium-, or low-yield zones. This technology is valuable to farmers because it
indicates which areas to apply the expensive fertilizers to achieve the highest crop
yield.
Light Detection and Ranging
Physics and astronomy
A worldwide network of observatories uses lidars to measure the distance to reflectors
placed on the moon, allowing the moon's position to be measured with mm precision
and tests of general relativity to be done. MOLA, the Mars Orbiting Laser Altimeter,
used a LIDAR instrument in a Mars-orbiting satellite (the NASA Mars Global
Surveyor) to produce a spectacularly precise global topographic survey of the red
planet.
In September, 2008, NASA's Phoenix Lander used LIDAR to detect snow in the
atmosphere of Mars.
In atmospheric physics, LIDAR is used as a remote detection instrument to measure
densities of certain constituents of the middle and upper atmosphere, such as
potassium, sodium, or molecular nitrogen and oxygen. These measurements can be
used to calculate temperatures. LIDAR can also be used to measure wind speed and to
provide information about vertical distribution of the aerosol particles.
At the JETnuclear fusion research facility, in the UK near Abingdon, Oxfordshire,
LIDAR Thomson Scattering is used to determine Electron Density and Temperature
profiles of the plasma.
Biology and conservation
LIDAR has also found many applications in forestry. Canopy heights, biomass
measurements, and leaf area can all be studied using airborne LIDAR systems.
Similarly, LIDAR is also used by many industries, including Energy and Railroad,
and the Department of Transportation as a faster way of surveying. Topographic maps
can also be generated readily from LIDAR, including for recreational use such as in
the production of orienteering maps.
In oceanography, LiDAR is used for estimation of phytoplanktonfluorescence and
generally biomass in the surface layers of the ocean. Another application is airborne
lidarbathymetry of sea areas too shallow for hydrographic vessels.
Light Detection and Ranging
In addition, the Save-the-Redwoods League is undertaking a project to map the tall
redwoods on California's northern coast. LIDAR allows research scientists to not only
measure the height of previously unmapped trees but to determine the biodiversity of
the redwood forest. Stephen Sillett who is working with the League on the North
Coast LIDAR project claims this technology will be useful in directing future efforts
to preserve and protect ancient redwood trees.
Military and law enforcement
Police officer using a hand-held LIDAR speed gun
One situation where LIDAR has notable non-scientific application is in traffic speed
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. LIDAR has the distinct advantage of being able to pick out one
vehicle in a cluttered traffic situation as long as the operator is aware of the
limitations imposed by the range and beam divergence.
LIDAR does not suffer from “sweep” error when the operator uses the equipment
correctly and when the LIDAR unit is equipped with algorithms that are able to detect
when this has occurred. A combination of signal strength monitoring, receive gate
timing, target position prediction and pre-filtering of the received signal wavelength
prevents this from occurring. Should the beam illuminate sections of the vehicle with
Light Detection and Ranging
different reflectivity or the aspect of the vehicle changes during measurement that
causes the received signal strength to be changed then the LIDAR unit will reject the
measurement thereby producing speed readings of high integrity. For LIDAR units to
be used in law enforcement applications a rigorous approval procedure is usually
completed before deployment. The use of many reflections and an averaging
technique in the speed measurement process increase the integrity of the speed
reading. Vehicles are usually equipped with a horizontally oriented registration plate
that, when illuminated, causes a high integrity reflection to be returned to the LIDAR
- despite the shape of the vehicle. In locations that do not require that a front or rear
registration plate is fitted, headlamps and rear-reflectors provide almost ideal retroreflective surfaces overcoming the reflections from uneven or non-compliant
reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which
cause concern that LIDAR is somehow unreliable.
Most traffic LIDAR systems send out a stream of approximately 100 pulses over the
span of three-tenths of a second. A "black box" proprietary statistical algorithm picks
and chooses which progressively shorter reflections to retain from the pulses over the
short fraction of a second.
Military applications are not yet known to be in place and are possibly classified, but
a considerable amount of research is underway in their use for imaging. Higher
resolution systems collect enough detail to identify targets, such as tanks. Here the
name LADAR is more common.
Utilizing LIDAR and THz interferometry wide area raman spectroscopy, it is possible
to detect chemical, nuclear, or biological threats at a great distance. Further
investigations regarding long distance and wide area spectroscopy are currently
conducted by Sandia National Laboratories.
Five LIDAR units produced by the German company Sick AG were used for short
range detection on Stanley, the autonomous car that won the 2005 DARPA Grand
Challenge.
A robotic Boeing AH-6 performed a fully autonomous flight in June 2010, including
avoiding obstacles using LIDAR.
Light Detection and Ranging
Vehicles
LIDAR has been used in Adaptive Cruise Control (ACC) systems for automobiles.
Systems such as those by Siemens and Hella use a lidar device mounted on the front
of the vehicle, such as the bumper, to monitor the distance between the vehicle and
any vehicle in front of it. In the event the vehicle in front slows down or is too close,
the ACC applies the brakes to slow the vehicle. When the road ahead is clear, the
ACC allows the vehicle to accelerate to a speed preset by the driver.
Imaging
3-D imaging is done with both scanning and non-scanning systems. "3-D gated
viewing laser radar" is a non-scanning laser radar system that applies the so-called
gated viewing technique. The gated viewing technique applies a pulsed laser and a
fast gated camera. There are ongoing military research programmes in Sweden,
Denmark, the USA and the UK with 3-D gated viewing imaging at several kilometers
range with a range resolution and accuracy better than ten centimeters.
Coherent Imaging LIDAR is possible using Synthetic array heterodyne detection
which is a form of Optical heterodyne detection that enables a staring single element
receiver to act as though it were an imaging array. This avoids the need for a gated
camera and all ranges from all pixels are simultaneously available in the image.
Imaging LIDAR can also be performed using arrays of high speed detectors and
modulation sensitive detectors arrays typically built on single chips using CMOS and
hybrid CMOS/CCD fabrication techniques. In these devices each pixel performs some
local processing such as demodulation or gating at high speed down converting the
signals to video rate so that the array may be read like a camera. Using this technique
many thousands of pixels / channels may be acquired simultaneously. In practical
systems the limitation is light budget rather than parallel acquisition.
LIDAR has been used in the recording of a music video without cameras. The video
for the song "House of Cards" by Radiohead is believed to be the first use of real-time
3D laser scanning to record a music video.
Light Detection and Ranging
Aerial Surveying - 3D mapping
Aerial LiDAR surveying from a paraplane operated by Scandinavian Laser Surveying
Airborne LIDAR sensors are used by companies in the Remote Sensing field to create
point clouds of the earth ground for further processing (e.g. used in forestry). A
common format for saving these points (with parameters like x, y, return, intensity,
elevation) is the LAS file format (see libLAS).
Light Detection and Ranging
ADVANTAGES
There are several advantages of LIDAR data. First, it is a very versatile technology
that has been used for atmospheric studies, bathymetric surveys, glacial ice
investigations, and numerous other applications. It is finding a lot of use in terrain
mapping. Here we see that this technology is a very cost effective method of terrain
data collection. It offers high precision and high point density data for DTM
modeling. Moreover, it has been shown to accelerate the project schedule, upwards to
30% because the DTM data processing can begin almost immediately. It is,
theoretically, not restricted to daylight nor cloud cover like aerial photography,
although if aerial imagery is being collected simultaneously, as it is commonly done,
then those limitation will affect the particular project. In coastal zones and forest
areas, LIDAR is considered as a superior data collection tool over conventional
photogrammetric techniques where it is extremely difficult to locate terrain points in
the imagery. LIDAR requires only one opening through a tree canopy to “see” the
ground whereas photogrammetry requires that the same ground point be visible from
two exposure stations. This would cut down on the amount of area identified as
“obscured terrain” on a contour map.
Photogrammetry is a mature science that is still undergoing technological advances.
The products derived from this mapping system are well received and the limitations
are understood. While LIDAR appears to be an excellent alternative to
photogrammetric mapping, there are several disadvantages to LIDAR when the two
technologies are compared.
Light Detection and Ranging
DISADVANTAGES
There are several disadvantages as well. While the data collection appears to be cost
competitive, the upfront cost of equipment acquisition is very significant, on the order
of $1 million. This could be a hard sell since amortization would have to be spread
over a very short period since the technology, like that of computers, will probably
experience a lot of change over the next two to three years. That is a lot of imagery to
collect over a short period of time. While LIDAR is an active system that can be,
theoretically, used 24 hours a day, it cannot be used above cloud cover or when fog,
smoke, mist, rain, or snow storms are present. Additionally, high winds and
turbulence will cause problems with the inertial system.
There are problems with data collected over water, which leads to suspect delineation
of water boundaries using LIDAR by itself. LIDAR systems are not capable of
determining break lines. Laser scan data are collected in a more or less regular
spacing pattern. In other words, it cannot be pointed on a specific feature. For
example, a 2 meter-wide ditch may not beshown on a LIDAR dataset with a spacing
of 5 meters. Thus, LIDAR data areoften augmented with break line data compiled
from photogrammetric methods.
Being a relatively new technology, standards have not been established that couldhelp
guide the user as to the quality of the results. There are a number of efforts
underway to alleviate this problem.
When elevation data are compiled from photogrammetric processes, the operatorhas a
“cartographic license” when selecting points for measurement. Contour
lines are generally smoothed to reflect the actual representation of the terrain. Alarge
boulder, as an example, may be a ground surface point captured by LIDARand the
resulting data may depict this as a high point in the terrain. Thephotogrammetric
operator would not use this point in the data collection processas a terrain point for
DEM generation.
Light Detection and Ranging
CONCLUSION
It is clearly evident that many within the GIS industry are looking at LIDAR as an
economical and accurate means of collecting both feature and terrain data. Indeed,
this technology is growing. Like any new technological tool, there are times when the
technology is misused. Just like GPS has not made conventional terrestrial surveying
obsolete, LIDAR will not soon supplant photogrammetric mapping as an economical
and accurate method of collecting data about features on the earth. As the
technologymatures, as new data processing techniques are developed, and as
standards are developed, it is safe to say that LIDAR will become an important data
collection methodology available to the user community.
Light Detection and Ranging
REFERENCES
1
Experimental Advanced Research Lidar', NASA.org. Retrieved 8 August 2007.
2
Mikkelsen, Torben& Hansen, Kasper Hjorth et al. Lidar wind speed measurements from a
rotating spinnerDanish Research Database & Danish Technical University, 20 April 2010.
Retrieved: 25 April 2010.
3
http://en.wikipedia.org/wiki/LIDAR
4
Ackermann, F., Airborne laser scanning present status and future expectations. ISPRS
Journal of Photogrammetry & Remote Sensing, Vol. 54, 1999.
5
Tom Paulson. 'LIDAR shows where earthquake risks are highest’, Seattle Post, April 18,
2001.