30-04-2011, 10:42 AM
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ABSTRACT
Light Detection and Ranging (LIDAR) have recently become the technologies of choice in mass production of Digital Elevation Models (DEMs), Digital Terrain Models (DTMs), and Triangulated Irregular Networks (TINs), referred to generically hereafter as DEMs. This paper presents lessons learned from LIDAR projects to date in various states. It addresses opportunities presented by LIDAR for generating DEMs as articulated by various user groups in the National Height Modernization Study. Finally, it summarizes actions required by the remote sensing community to establish LIDAR as standard tools, with established standards, for generating digital elevation data for the new millennium.
INTRODUCTION
LIDAR (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 geometrics, archaeology, geography, geology, geomorphology, seismology,forestry, remote sensing and atmospheric physics.[1] 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 but is misleading because it uses laser light and not the radiowaves that are the basis of conventional radar.
GENERAL DESCRIPTION
The primary difference between LIDAR and RADAR is 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 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 wavelength-dependent 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 downward-looking LIDAR instruments mounted in aircraft or satellites. A recent example is the NASA Experimental Advanced Research LIDAR.
A basic LIDAR system involves a laser range finder reflected by a rotating mirror (top). The laser is scanned around the scene being digitized, in one or two dimensions (middle), gathering distance measurements at specified angle intervals (bottom).
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: MICROPULSE LIDAR 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.).
On a functional level, LiDAR is typically defined as the integration of three technologies into a single system capable of acquiring data to produce accurate digital elevation models (DEMs). These technologies are lasers, the Global Positioning System (GPS), and inertial navigation systems (INS). Combined, they allow the positioning of the footprint of a laser beam as it hits an object, to a high degree of accuracy.
Lasers themselves are very accurate in their rang¬ing capabilities, and can provide distances ac¬curate to a few centimeters. The accuracy limi¬tations of LiDAR systems are due primarily to the GPS and IMU (Inertial Measurement Unit) compo¬nents. As advancements in commercially avail¬able GPS and IMUs occur, it is becoming possible to obtain a high degree of accuracy using LiDAR from moving platforms such as aircraft.ALiDAR system combines a single narrow-beam laser with a receiver system. The laser produces an optical pulse that is transmitted, reflected off an object, and returned to the receiver. The re¬ceiver accurately measures the travel time of the pulse from its start to its return. With the pulse trav¬elling at the speed of light, the receiver senses the return pulse before the next pulse is sent out. Since the speed of light is known, the travel time can be converted to a range measurement. Combining the laser range, laser scan angle, la¬ser position from GPS, and laser orientation from INS, accurate x, y, z ground coordinates can be calculated for each laser pulse.
Laser emission rates can be anywhere from a few pulses per second to tens of thousands of pulses per sec¬ond. Thus, large volumes of points are collected. For example, a laser emitting pulses at 10,000 times per second will record 600,000 points every minute. Typical raw laser point spacing on the ground ranges from 2 to 4 meters.
Some LiDAR systems can record “multiple re¬turns” from the same pulse. In such systems the beam may hit leaves at the top of tree canopy, while part of the beam travels further and may hit more leaves or branches. Some of the beam is then likely to hit the ground and be reflected back, ending up with a set of recorded “multiple returns” each having an x, y, z position. This fea¬ture can be advantageous when the
application calls for elevations for not only the ground, but for tree or building heights.
As surface types and characteristics vary and change the laser beam’s reflectivity, then the ability of the LiDAR to record the return signals changes. For example, a laser used for topo¬graphic applications will not penetrate water, and in fact records very little data even for the surface of the body of water. Where the appli¬cation calls for a laser to penetrate water to de¬termine x, y, z positions of undersea features, then a slightly different variation of LiDAR technology is used.
ABSTRACT
Light Detection and Ranging (LIDAR) have recently become the technologies of choice in mass production of Digital Elevation Models (DEMs), Digital Terrain Models (DTMs), and Triangulated Irregular Networks (TINs), referred to generically hereafter as DEMs. This paper presents lessons learned from LIDAR projects to date in various states. It addresses opportunities presented by LIDAR for generating DEMs as articulated by various user groups in the National Height Modernization Study. Finally, it summarizes actions required by the remote sensing community to establish LIDAR as standard tools, with established standards, for generating digital elevation data for the new millennium.
INTRODUCTION
LIDAR (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 geometrics, archaeology, geography, geology, geomorphology, seismology,forestry, remote sensing and atmospheric physics.[1] 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 but is misleading because it uses laser light and not the radiowaves that are the basis of conventional radar.
GENERAL DESCRIPTION
The primary difference between LIDAR and RADAR is 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 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 wavelength-dependent 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 downward-looking LIDAR instruments mounted in aircraft or satellites. A recent example is the NASA Experimental Advanced Research LIDAR.
A basic LIDAR system involves a laser range finder reflected by a rotating mirror (top). The laser is scanned around the scene being digitized, in one or two dimensions (middle), gathering distance measurements at specified angle intervals (bottom).
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: MICROPULSE LIDAR 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.).
On a functional level, LiDAR is typically defined as the integration of three technologies into a single system capable of acquiring data to produce accurate digital elevation models (DEMs). These technologies are lasers, the Global Positioning System (GPS), and inertial navigation systems (INS). Combined, they allow the positioning of the footprint of a laser beam as it hits an object, to a high degree of accuracy.
Lasers themselves are very accurate in their rang¬ing capabilities, and can provide distances ac¬curate to a few centimeters. The accuracy limi¬tations of LiDAR systems are due primarily to the GPS and IMU (Inertial Measurement Unit) compo¬nents. As advancements in commercially avail¬able GPS and IMUs occur, it is becoming possible to obtain a high degree of accuracy using LiDAR from moving platforms such as aircraft.ALiDAR system combines a single narrow-beam laser with a receiver system. The laser produces an optical pulse that is transmitted, reflected off an object, and returned to the receiver. The re¬ceiver accurately measures the travel time of the pulse from its start to its return. With the pulse trav¬elling at the speed of light, the receiver senses the return pulse before the next pulse is sent out. Since the speed of light is known, the travel time can be converted to a range measurement. Combining the laser range, laser scan angle, la¬ser position from GPS, and laser orientation from INS, accurate x, y, z ground coordinates can be calculated for each laser pulse.
Laser emission rates can be anywhere from a few pulses per second to tens of thousands of pulses per sec¬ond. Thus, large volumes of points are collected. For example, a laser emitting pulses at 10,000 times per second will record 600,000 points every minute. Typical raw laser point spacing on the ground ranges from 2 to 4 meters.
Some LiDAR systems can record “multiple re¬turns” from the same pulse. In such systems the beam may hit leaves at the top of tree canopy, while part of the beam travels further and may hit more leaves or branches. Some of the beam is then likely to hit the ground and be reflected back, ending up with a set of recorded “multiple returns” each having an x, y, z position. This fea¬ture can be advantageous when the
application calls for elevations for not only the ground, but for tree or building heights.
As surface types and characteristics vary and change the laser beam’s reflectivity, then the ability of the LiDAR to record the return signals changes. For example, a laser used for topo¬graphic applications will not penetrate water, and in fact records very little data even for the surface of the body of water. Where the appli¬cation calls for a laser to penetrate water to de¬termine x, y, z positions of undersea features, then a slightly different variation of LiDAR technology is used.
