In Lessons 1 and 2, you learned about the basic workings of a scanning lidar sensor and georeferencing systems. You should now have an appreciation for the fact that a lidar mapping system actually comprises the sensor itself, the platform upon which the sensor is mounted, and the direct georeferencing equipment that enables conversion of individual lidar ranges into meaningful geographic coordinates. Lesson 3 will discuss the major classes of platforms used for deployment of lidar sensors:
The choice of platform depends largely on the application: spaceborne systems map remote areas of the globe for scientific purposes; airborne platforms are effective for topographic mapping and engineering applications; and ground-based platforms are frequently used to produce detailed 3D models of buildings, bridges, streetscapes, factories, and other man-made infrastructure. The sensors used on these various platforms all operate according to the basic principles outlined in Lesson 1; however the design, appearance, and technical specifications of each system will be optimized for the platform and applications. For example, lidars used for bathymetric mapping use a blue-green wavelength laser, rather than an infrared wavelength, in order to penetrate shallow water.
In this lesson, you will be exposed to many examples of platform/sensor systems, and you will learn how they are optimized to produce relevant information for each application domain. In Lesson 3, we will delve more deeply into the georeferencing (using GPS and IMU) of airborne topographic lidar data, the processing and use of which is the primary focus of this course.
At the successful completion of this lesson you should be able to:
The simplest airborne lidar system is a laser rangefinder, or laser altimeter, that measures the distance between the plane and the ground directly below. The result is a profile line on the ground aligned with each individual flight line, as depicted by the animated Figure 2.01. The earliest laser profiling systems predated GPS; they measured the relative distance between the plane and the ground, and a standard barometric altimeter provided the altitude of the aircraft relative to mean sea level. These systems are of little use in wide-area topographic mapping; however, they can be quite useful (and economical) for monitoring small changes of elevation over time across specific areas of interest, such as glaciers, ice sheets, and research forests. Shan and Toth (2008) also describe an interesting use of a laser profiler to calibrate less-accurate elevation data from the GeoSAR interferometric synthetic aperture radar system operated by Fugro EarthData.
Systems comprising a) a laser ranging unit, b) a scanning mechanism such as an oscillating mirror or rotating prism, and c) a direct georeferencing system are classified as laser scanners as opposed to laser profilers or altimeters. Chapter 7 of Maune (2007) describes a number of laser scanning technologies. Topographic and bathymetric mapping systems (discussed briefly on page 7 on this lesson) use scanning systems exclusively due to their ability to seamlessly cover large swaths of ground, as demonstrated in the animated Figure 2.02.
Aircraft are by far the predominant platform for topographic lidar mapping systems. They are capable of operating over a wide range of altitudes to accommodate project-specific needs for accuracy, point density, and broad area coverage. Aircraft can be deployed wherever and whenever weather conditions are favorable. For example, clouds often appear and dissipate over the target area over a period of several hours during a given day. Aircraft on site can respond on short notice to take advantage of clear conditions, while satellites are locked into a schedule dictated by orbital parameters. Aircraft can also be deployed in small or large numbers, making it possible to collect imagery seamlessly over an entire county or state in a matter of days or weeks simply by having lots of planes in the air at the same time. Typical point densities on the ground range from 1 point per 3 - 5 meters up to several points per square meter at accuracies on the order of 10-15 centimeters.
In Renslow (2012), Chapter 3, you will be introduced to the primary aircraft platforms used for photogrammetric mapping and commercial airborne remote sensing, including lidar. These aircraft range from the very small, slow, and low flying Helio Courier (Figure 2.03), to the Cessna Conquest (Figure 2.04), a twin-engine turboprop jet capable of flying at altitudes up to 35,000 feet. The operating ceiling for an aircraft is defined in terms of altitude above mean sea level. It is important to remember this distinction when planning a remote sensing project, where scale and altitude are defined above the mean terrain (AMT). For example, if a project's requirements state that lidar is to be captured at an average altitude of 5,000 feet above the ground, and the average elevation of the project area is 1,500 feet above sea level, then the aircraft is must actually operate at 6,500 feet above sea level. In very mountainous terrain, such as the Rocky Mountain range of the western US or the Swiss Alps, a pressurized platform like the Cessna Conquest would be suitable for meeting operating altitude requirements. Modifications to the fuselage and power system to accommodate a lidar instrument and data storage system can be more expensive than the cost of the aircraft itself. While the planes used in airborne lidar mapping are fairly common, choosing the right aircraft to invest in requires a thorough understanding of the applications for which that aircraft is likely to be used over its lifetime.
Helicopters' implementations of lidar are similar to fixed-wing. The major advantage of a helicopter is the ability to fly low, slow, and to closely follow a complex corridor, such as a power line or railroad track. Much higher point densities can be achieved from the same basic lidar sensor simply because the scanner is moving much more slowly across the target area. Helicopter-based systems can collect tens to hundreds of points per square meter on the ground and can achieve vertical accuracies of a centimeter or better. The following figures show examples of lidar data collected from rotary-wing platforms.
In general, satellites are placed in one of three types of orbits around the Earth: geostationary, polar, or sun-synchronous. These orbits are fixed; a single satellite orbit can be adjusted slightly to maintain consistency over time, but it cannot be changed from one type to another. The type of orbit determines the design of the sensor, its altitude with respect to the Earth, and its instantaneous field of view (the area on the Earth which can be viewed at any particular moment in time).
Geostationary (or geosynchronous) satellites appear to be stationary with respect to the Earth, as shown in Figure 2.08. They must be placed at a very high altitude (~ 36,000 km) in order to produce an orbital period equal to the period of Earth's rotation. Any sensor onboard a geosynchronous satellite is viewing the same area of the Earth at all times, and because of the high latitude, this is usually a very large area. Communications and weather satellites are the most common examples of geosynchronous orbits. In general, imaging and mapping satellites are not geosynchronous because a) the resolution of imagery or mapping data acquired from this great distance would be very coarse, b) most satellite remote sensing applications are based on the availability of world-wide coverage, and c) passive remote sensing systems require the Sun's illumination. While lidar, an active sensor, does not require daylight, the first two reasons make geosynchronous lidar profilers impractical.
Polar orbiting satellites pass above (or nearly above) each of the Earth's poles, and pass over the equator at a different longitude, on each revolution, as illustrated in Figure 2.09. A polar orbiting satellite eventually sees every part of the Earth's surface, which is highly desirable for remote sensing applications, including lidar.
Sun-synchronous satellites are deployed as a special case of polar orbits, where each successive orbit crosses the equator at intervals of 15 degrees of longitude and precessing around the entire globe once per day. The satellite will pass over every location on earth at the same local solar time. These are the most common orbits for remote sensing satellites in order to provide illumination for passive sensors. Active sensors such as radar and lidar may not require the Sun's illumination for imaging, but they do rely on solar energy as a source of power. This reason and the global coverage provided by a polar orbit make a sun-synchronous orbit a good choice for a spaceborne laser profiler.
In 2003, NASA's Ice Cloud and Land Elevation Satellite (ICESat) was launched, carrying the Geoscience Laser Altimeter System (GLAS), to collect information about the polar ice caps, global cloud cover, vegetation canopy, and other parameters of scientific interest. GLAS uses the same 1024 nm laser found in airborne topographic lidar systems, but its footprint on the ground is about 65 meters as compared to a meter or less for an airborne scanner. It is in a sun-synchronous orbit at 700 km above the Earth's surface and profiles the same location on the Earth every 91 days (UCAR, 2009).
The characteristics of a lidar system suitable for spaceborne operation are significantly different than those for an airborne system (Shan and Toth, 2008); large and heavy power supplies and optics are required to send a laser pulse and receive its reflection over the hundreds of kilometers separating the sensor and the Earth. Due to these limitations, all operational space-based laser sensors to date are profilers, rather than scanners. Although the sensor design may be quite different, the basic principles of lidar ranging are the same, whether from a spaceborne or airborne platform. The optional reading assignment in Shan and Toth enumerates technical design specifications of the GLAS and MBLA spaceborne laser profilers for ice sheet monitoring and global vegetation characterization.
The design of a sensor destined for a satellite platform begins many years before launch and cannot be easily changed to reflect advances in technology that may evolve during the interim period. While all systems are rigorously tested before launch, there is always the possibility that one or more will fail after the spacecraft reaches orbit. The sensor could be working perfectly, but a component of the spacecraft bus (attitude determination system, power subsystem, temperature control system, or communications system) could fail, rendering a very expensive sensor effectively useless. The financial risk involved in building and operating a satellite sensor and platform is considerable, presenting a significant obstacle to the commercialization of space-based lidar.
One obvious advantage satellites have over aircraft is global accessibility; there are numerous governmental restrictions that deny access to airspace over sensitive areas or over foreign countries. There are obvious physical limitations on the operation of aircraft over the remote ice caps and polar regions, where laser profiling can make important contributions to scientific research. Satellite orbits are not subject to these restrictions, although there may well be legal agreements to limit distribution of data collected over particular areas of the globe.
University Center for Atmospheric Research (2009). Facilities Assessment Database. http://www.eol.ucar.edu/fadb/resource/show/1021. Last accessed 18 September 2009.
Ground-based, or terrestrial, lidar systems are used for close-range, high-accuracy applications, such as bridge and dam monitoring, architectural restoration, facilities inventory, crime and accident scene analysis, landslide and erosion mapping, and manufacturing. They have even been used to create novel music videos and movie special effects. A combination of airborne and terrestrial lidar can provide realistic 3D models of built-infrastructure with accurate rooftops and realistic building facades.
Using an infrared or green wavelength laser, ground-based lidars pulse at rates up to 1000 Hz, and can map objects from about 1 meter up to 1000 meters away with accuracies on the order of millimeters to a few centimeters. The accuracy of individual points can be affected by atmospheric conditions, distance to the target, angle of incidence of the laser pulse upon the target, and the reflectivity of the target surface. Very shiny, polished surfaces and very black surfaces that absorb nearly all incident light are difficult to image.
Ground-based lidars are either static (on a stationary platform such as a tripod or mast) or dynamic (on a moving vehicle). It has been incorporated into surveying and metrology instruments and is often employed in mobile mapping systems. In a static implementation, a GPS/INS georeferencing system is not needed. The lidar is set up over a known point, and the scan angles for each point are recorded in the data set. Reference points on the target surface can also be surveyed to provide additional georeferencing control. In a dynamic implementation of ground-based lidar, GPS/IMU is utilized to provide georeferencing, just as it would be on an airborne platform. Mobile mapping systems are often used to develop 3D streetscapes in cities, where GPS signals can be blocked by tall buildings or affected by error-inducing "multipath." Mobile mapping systems employ additional motion-detecting sensors to provide corrections to the GPS/IMU in these applications.
Many ground-based lidars use the simple principle of laser pulse rangefinding introduced earlier in this lesson. These systems pulse at lower frequencies and can measure distances of several hundred meters with centimeter-level accuracy. Another technique, called "phase-differencing," is used in systems intended for very close range (less than 50 meters) work with millimeter-level accuracy. For those who are familiar with GPS, the difference between pulse rangefinding and phase differencing is analogous to the difference between GPS code-phase and carrier phase signal processing. For the interested student, these concepts are explained with more detail in Shan and Toth. We will only be dealing with pulse rangefinding lidars for the remainder of this course.
Three types of scanning systems are employed in ground-based lidar:
Ground-based lidars can also be classified according to operational range:
Bathymetric lidar is briefly described in this lesson, but, due to lack of time, will not be given further treatment in this course. Many engineering and environmental studies require information about the surface underlying streams, rivers, lakes, bays, and shallow coastal waters, and questions about the capabilities of bathymetric lidar to provide this critical information are frequently asked. The student who completes this course should be able to discuss the capabilities and limitations of bathymetric lidar in the context of these common topographic mapping applications.
Lidars designed for mapping underwater use a blue-green laser that can penetrate water and provide returns of underwater objects or the bottom. Low-flying aircraft equipped with GPS/IMU and a pulsed laser scanner are the platform of choice for this application. The data are used to support navigation, military operations, and environmental and recreational needs. Bathymetric lidar systems are often flown simultaneously with digital cameras and hyperspectral sensors to gather additional information about water quality and bottom composition. Water clarity and depth are the most significant limiting factors; in the clearest water, penetration down to 50 meters can be achieved. The rule of thumb is penetration to 2-3 times the Secchi depth (a traditional measure of water clarity based on the ability of the human eye to detect a submerged black or white disc).
Despite the limitations, lidar fills an important gap in collection of critical near-shore bathymetry. Ship-based sonars can collect depth data in deep water, but cannot operate in shallow, near-shore waters where lidar is most effective. As shown in Figure 2.13, combining topographic and bathymetric lidars on the same platform makes it possible to collect this near-shore bathymetry, along with direct observation of the coastline, beaches, and dunes.
There are many unique complications that preclude widespread commercial use of this technology. Nautical charts depict depth below some "average" water surface, while lidar measures absolute elevation of the reflective surface based on GPS and IMU. The water surface elevation must be measured at the same time to produce meaningful depths; this is illustrated in Figure 2.14 and accomplished using processing methods described in the textbook. The measured water surface elevation must then be related to the vertical datum used for nautical charting. Another obvious practicality is the need to coordinate data acquisitions missions with tidal conditions. While, due to cost and operational limitations, there are only a few bathymetric lidar systems in operation around the world, they supply critical information that cannot be readily acquired by other methods.
This activity should be worked on after you have completed all of the online and textbook readings for this lesson. We recommend you take the following approach:
The ASPRS monthly journal, Photogrammetric Engineering and Remote Sensing, contains a regularly published column, written by Dr. Qassim Abdullah, called "Mapping Matters." Each month, Dr. Abdullah answers a reader's question about the creation and use of imagery and elevation data for use in GIS applications.
Browse the Mapping Matters Archive (2007 - present) on the ASPRS website. Find a Q&A entry in the archive that addresses a sensor, platform, or georeferencing issue related to the course material. It may be a question or a problem you have encountered in your professional work or one that simply seems interesting to you. Summarize the question for the rest of the class in the Lesson 3 Graded Discussion Forum. Explain your interest in the question and discuss, in your own words, how the answer relates to the material taught in this lesson. What did you learn in this lesson that helped you understand Dr. Abdullah's response? How might this new bit of knowledge help you in your current or future work? What was surprising to you about either the question or the answer? If you didn't fully understand the answer, what questions remain for you?
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