PE&RS November 2001

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Laser Altimetry: From Science to Commercial Lidar Mapping

Introduction
Laser altimetry, more commonly referred to in the commercial sector as lidar mapping1 , is becoming a commonplace operational tool in the fields of remote sensing, photogrammetry, surveying and mapping. Laser altimetry is capable of rapidly generating dense, accurate, digital models of the topography and vertical structure of a target surface. It is an attractive tool for data end users in various application areas since the cost to produce the elevation data, point for point, can be significantly less than other forms of traditional data collection. For any application with a need for high density, high accuracy elevation models, laser altimetry offers unique technical capabilities, lower field-operation costs, and reduced post-processing time and effort compared to traditional survey methods.

The commercial deployment of laser altimetry has grown rapidly since 1995 due to the increasing availability of commercial off-the-shelf sensors, advancements in the design and capabilities of the sensors themselves and an increased awareness by end users and contracting agencies of the advantages of using lidar technology for elevation data capture. As a result, the commercial sector has been experiencing strong growth, which in turn has spurred further developments by the research sector and even greater demand for the data products from end users. While the adoption of laser altimetry by the commercial sector has been increasing rapidly, the transition from science to an operational remote sensing tool has not been smooth. Early failures and poor expectation management by instrument vendors and data providers have generated skepticism about the technology. This skepticism needs to be addressed and end users need to be better educated about the strength of the underlying science as well as the capabilities of today’s commercial lidar mapping industry if further adoption is to be encouraged.

The Science
The use of lasers as remote sensing instruments has an established history going back more than 30 years. Townes and Schawlow first put the theory of the optical maser or laser forward in 1958. Maiman demonstrated the first successful laser – a ruby laser – in 1960. Through the 1960s and 70s various experiments demonstrated the power of using lasers in remote sensing including lunar laser ranging, satellite laser ranging, atmospheric monitoring and oceanographic studies. During the 1980s, laser altimetry – essentially the measurement of height using a laser rangefinder – developed as airborne instruments such as NASA’s Atmospheric Oceanographic Lidar (AOL)² and Airborne Topographic Mapper (ATM)³ were deployed. Today, laser altimetry has been successfully demonstrated from a variety of airborne platforms and from near-Earth orbit during the Shuttle Laser Altimeter (SLA) missions⁴. NASA currently has two satellite missions planned that will deploy laser altimetry; the Vegetation Canopy Lidar mission (VCL)⁵ and the Geosciences Laser Altimeter (GLAS)⁶. Laser altimetry has also been used to provide us with spectacular images and detailed maps of Mars via the Mars Observer Laser Altimeter (MOLA)⁷.

Today, two distinct techniques in laser altimetry are being actively investigated; small footprint, time-of-flight laser altimetry and large footprint, waveform-digitizing techniques that analyze the full return waveform to capture a complete elevation profile within the target footprint. The term lidar – LIght Detection and Ranging – refers to the optical equivalent of radar and is often used in place of the more specific term, laser altimetry. In time-of-flight lidar instruments the distance or range from the sensor to any reflective surface beneath the platform is determined by measuring the elapsed time between the generation and return of each laser pulse. By scanning the laser across the path of the platform, a wide swath of laser range data is captured along the flight path. These laser ranges are combined with platform position and orientation information during post-processing to create a geo-referenced point cloud that is essentially a digital 3D model of the surface beneath the platform. In the commercial sector, small footprint, time-of-flight lidar mapping has established itself as a robust, cost-effective operational tool, but the technology is still developing and the commercial sector is still developing appropriate business models for effectively delivering this type of data to the end-users.

An alternative technique to the basic time-of-flight method involves recording the entire return waveform from the laser pulse. The difference between these two approaches is demonstrated in Figure 1. By capturing the full return waveform, detailed information on the entire vertical structure within the laser footprint is obtained and ground topography can be detected even with canopy openings of only a few percent. This type of waveform characterization of the complete canopy profile can have significant value in scientific research and may have commercial importance in the forestry industry. Perhaps more important for the commercial sector, waveform capture from large footprint sensors has been demonstrated as an effective method of determining the ground surface underneath even the densest canopy.

Figure 1. NASA SLICER Return Waveform Capture. Incident pulses of laser energy reflect off various portions of the canopy, resulting in a return waveform or “echo” where the amplitude of the pulse at a given height is a function of the canopy structure and the last large-amplitude spike is the ground return. Courtesy of M. Lefsky adapted from Blair/Harding NASA GSFC.

Experimental flights of NASA’s Laser Vegetation Imaging Sensor (LVIS)⁸ were able to successfully and accurately record the topography beneath dense rainforest in Costa Rica9 . Small footprint time-of-flight sensors become suspect in dense, complex canopy as not only must they successfully penetrate through the gaps in the canopy to the ground surface, but post-processing of the point cloud must employ filtering algorithms to correctly classify those returns that are from the ground from those that are from canopy or other features. Even the most advanced algorithms have difficulty accurately extracting the ground surface with 100 percent fidelity, especially in areas of low, dense ground cover, significant relief or where sudden changes in topography such as gullies or sharp grade breaks mimic man-made features. The complex interaction of the laser pulse with the distributed vertical target is also a concern when trying to determine the point of reference for a time-of-flight sensor. As a result, waveform-capture sensors offer a powerful alternative approach to map the bare earth beneath dense canopy. While full waveform capture is already being used in commercial laser bathymetry, its use in topographic applications is still limited to scientific and research programs.

Commercial Lidar Mapping: The Situation Today
Prior to 1995, laser altimetry was generally conducted using custom-designed sensors operated by research groups or built by commercial survey firms to exploit niche markets. Such custom-developed sensors required organizations to dedicate significant resources to the effort and develop expertise in various normally unrelated disciplines. The majority of these efforts were based on single prototype designs, limiting their ability to create and service a broad, sustainable, sector-wide demand for the technology. However, since 1995 a commercial off-the-shelf (COTS) instrument market has developed which has removed many of these constraints for organizations wanting to incorporate laser altimetry into their operations. The availability of COTS sensors has increased access and driven a much more rapid adoption of the technology. The pioneering work of groups such as NASA, NOAA and the University of Stuttgart has also been an important factor in the recent acceptance of the technology. As a result, laser altimetry is in the process of establishing itself as a robust, cost-effective operational tool, although application-specific methodologies are still being developed in many areas.

The basic principles and design constraints of laser altimetry are well known^{10 ,11 ,12} , however there are still significant variations in design from instrument to instrument, especially across custom-designed sensors. The general characteristics and specifications of the current generation of commercial systems used for topographic mapping can be summarized as follows:

As of July 1, 2001, there were approximately 75 organizations worldwide operating approximately 60 sensors for commercial applications with additional sensors being deployed by research groups. Growth rates in the commercial sector in terms of installed instrument base have been averaging about 25 percent per year since 1998, with projections for an installed instrument base of 150 – 200 sensors by 2005¹³ . There are also a growing number of value-added resellers and product developers that include lidar mapping and lidar data analysis as an integral part of their activities. Strong evidence of the growing demand for laser altimetry in the commercial sector can be seen in recent mergers and acquisition that have focused on lidar or had a strong lidar component. These include 3Di’s acquisition in May 2000 of EagleScan Inc. of Denver, Colorado, an early entrant in the lidar remote sensing market. The acquisition of EagleScan added lidar and other advanced remote sensing capabilities to 3Di’s existing mainstream mapping capabilities. On October 10, 2000, MacDonald, Dettwiler and Associates Ltd. (MDA), parent company of Triathlon Ltd., acquired Atlantic Technologies, LLC of Huntsville, Alabama. Atlantic Technologies had been offering lidar services for approximately a year prior to the acquisition and this in-house lidar capability clearly was a major component of the acquisition. MDA gave further evidence of the acceptance of lidar technology in August by announcing its subsidiaries had reported considerable success offering customers lidar mapping resulting in the signing of $3.75 million (CDN) in contracts during the previous two months. More recently, on April 27, 2001, the Sanborn Map Company, Inc., the oldest mapping company in the United States, acquired substantially all the assets of Analytical Surveys Inc.’s Colorado Springs, Colorado-based land mapping office. This included ASI’s lidar-mapping assets and staff.

While the above mergers are examples of major mainstream mapping firms moving to acquire lidar mapping capability and expertise, perhaps more indicative of the growing adoption of lidar technology by the mainstream was the recent acquisition of Azimuth Corporation by LH Systems on May 4, 2001. LH Systems, a subsidiary of Lecia GeoSystems, acquired the Westford, Massachusetts manufacturer of the AeroScan lidar system. The AeroScan sensor is the second most numerous COTS lidar instrument after the Optech ALTM brand. The AeroScan sensor, re-branded as the LH Systems ALS40 Airborne Laser Scanner, will join the LH Systems product suite of sensors, including film and digital aerial cameras for the acquisition, processing, and maintenance of precise information from imagery. For many advocates of laser altimetry, the entrance of an established, well-respected and well-known camera manufacturer such as LH Systems adds significant credibility to the tool and bodes well for wider adoption of lidar technology.

Pricing
In the transition from science to operational tool the issue of pricing becomes critical to achieving widespread adoption of a new remote sensing technology by mainstream users. Price points need to be established that reflect a reasonable value proposition for both the data provider and the data end user, while accurately accounting for differences in product definition, accuracy and quality, and the availability of competing or alternative methods of data capture. Unfortunately commercial pricing for products derived from laser altimetry, such as bare earth DTMs, is still difficult to generalize due to the variety of data products that can be produced, the lack of a defined and accepted benchmark product around which to compare pricing, the many input and delivery variables on a given project and the various applications that can be addressed by the data. In addition, pricing strategies in the past have varied considerably between data providers. Some commercial operators quote on a margin basis, calculating their costs to complete the project and then adding their profit margins. Others use value strategies, calculating the data value based on existing tools and pricing at a discount. Sometimes firms will price strategically, determining price based on business drivers such as minimizing idle capacity, maximizing regional coverage, capturing market share, excluding competitors, servicing favored clients, pricing to approved contracting agency budgets, or leveraging out revenues and cash flow. In the past, many providers have used different pricing strategies at different times or sometimes a combination of several strategies, with the resulting bids appearing to the contracting organization to be haphazard rather than based on a rational, predictable pricing model. As with other remote sensing tools, the availability of public-domain data sets generated by government or university research groups also complicates the issue as does competition in the private sector by research or government groups using sensors acquired or developed with public funds. A user base that is less experienced with lidar technology than with other more established methods also complicates the issue and raises the risk of exploitation of less educated end users.

However, with the gradual maturity of the lidar sector along with increased experience with the technology by end users, it can be anticipated that prices will stabilize. The primary cost drivers for a given level of lidar-derived DEM include project scope, mobilization and demobilization costs, project boundary shape and project deliverables. Secondary factors include terrain relief, vegetation, culture, features, access and extent of available ground control sites, even the season of the year. As a result, there are various price ranges an end user can expect to see when requesting quotes for lidar DEMs. At the time of this writing, lidar DEM pricing in the domestic U.S. market, for a data product capable of supporting 2' contour-interval mapping, can be found ranging from less than $1.00 U.S. per acre to in excess of $250 U.S. per acre. There are several published benchmark lidar contracts in the public domain – see for example the Puget Sound Lidar Consortium; Kitsap County Survey¹⁴ and the North Carolina - Floodplain Mapping Program¹⁵ – which can act as pricing guidelines for large area lidar surveys. In the twelve months from July 2000 to the end of June 2001, the general range for large area projects of 1000 sq. miles or more in the domestic U.S. market was running $0.50 - $1.00 U.S. per acre for the basic lidar DEM product, largely depending on the amount of manual post-processing to be performed.

Unfortunately for end users, the lidar pricing is often buried in a larger per acre cost that includes additional value-added components such as imagery, planimetrics, or contouring, making isolated lidar costing difficult to determine if not specifically requested. Published pricing on smaller projects, ranging down to several hundred acres, is much more difficult to obtain. Minimum entry price points for smaller projects in the domestic U.S. range from less than $7,000 US to upwards of $25,000 US depending upon the data provider. For even smaller projects, on the order of tens of acres, the per acre cost becomes significantly higher due the fixed sunk costs of mobilizing the lidar sensor. For many common mapping applications this increase pushes the price well beyond the level that can be justified by the end user, leaving lidar beyond the reach of most of these smaller mapping projects. So while lidar projects can enjoy significant cost advantages on larger projects as compared with photogrammetric methods, the per unit cost of smaller projects can be significantly higher. For this reason, the minimum breakeven point for employing lidar varies significantly across existing data providers, ranging upwards from a minimum of 1000 acres, depending upon the location of the project and the data provider chosen. Although no rigorous analysis has been published, it appears the present market pricing from leading data providers offers lidar DEM data at a 10 to 15 percent discount from photogrammetrically-derived DEMs of equivalent accuracy, once the breakeven point is surpassed.

Future Developments: 2001 — 2005
Laser altimetry has been studied since the 1960s and it continues to be an active area of research and development even today. No doubt many of these on-going developments will be incorporated into the commercial lidar sector as they are proven to be feasible and cost-effective. Research in many different areas related to sensor design is on-going and significant effort is being applied to data analysis techniques, however it is possible to highlight five key areas that will have significant impact in the next five years, from both a commercial and academic point-of-view:

Higher Density Data
For a given sensor the achievable point density on the ground is a direct function of the operating altitude, platform speed, scan angle, scan speed and laser repetition rate of the sensor. For a given set of operating conditions laser repetition rate becomes the primary figure of merit, with higher laser repetition rates corresponding to higher data density on the ground or conversely an equivalent data density from a more cost-effective altitude. As a result, repetition rate has become a prime differentiating factor in the marketing of both sensors and data collection services, despite a clearly documented, requirements-driven analysis of the necessary density needed to support different mapping products or applications. Figure 2 shows the historical growth in repetition rate for small footprint, time-of-flight COTS sensors, with rates increasing from less than 5 kHz in 1995 to 35 kHz today with a 50 kHz sensor already being field-tested. Extrapolating this growth curve implies that sensors with 100 kHz or higher data collection rates will be the benchmark by 2005, a conclusion supported by developments in the underlying laser technology. Such sensors will generate data sets 4x denser than currently available and 20x denser than those that have been routinely available since 1996. Higher repetition rates should allow cost-effective generation of extremely detailed DEMs over wide areas, providing an exciting new view of the Earth for a variety of research and commercial applications.

Commercial Off-The-Shelf Software
The emergence of robust, reliable software tools that are available to the entire community will be one of the most significant areas of change in the next five years. Currently, the vast majority of the processing, manipulation and classification of lidar data is conducted using proprietary software developed independently by researchers, the data providers, or provided by the sensor manufacturer to its clients but not available as a separate package (e.g. Optech’s REALM software). The current situation presents a significant barrier to end users as lidar processing is presented as a “black box” with limited insight into the actual manipulation of the data and a very limited ability of the end user to recreate, reclassify, manipulate or modify the data sets they are provided. The fact that few of these proprietary classification algorithms have been published or opened up to peer review is also a concern among academics and researchers. However, by its very nature as open-format data representing a well-defined geospatial point cloud, lidar data is relatively easy to manipulate by third-party software. To date there are only a few software products on the market that can efficiently handle the large point densities generated by state-of-the-art lidar sensors – easily in excess of 100s of millions of points for even moderately sized projects – but this situation is changing rapidly. Third-party products specifically designed for manipulating lidar data are starting to appear and existing mainstream software developers, such as ESRI, are moving to integrate lidar data manipulation into their existing product suites. The availability of appropriate software tools for the entire end user community will eventually replace the proprietary, black box solutions common today. This will open up the post-processing workflow and fundamentally change the existing value chain, presenting a serious challenge to the “status quo” of the first generation of commercial data providers. By 2005 the availability of a suite of commercial off-the-shelf software tools will shift the primary product requested of lidar data providers from bare earth DTMs to the more basic geo-referenced, all-points laser point cloud.

Waveform Capture
While the majority of the laser altimetry projects done in the commercial sector today are based on time-of-flight systems that minimize footprint size and maximize repetition rate, a significant effort is being undertaken by research groups, such as NASA, to develop waveform digitizing sensors that capture the full return waveform. Airborne test beds incorporating this technique, such as NASA’s Laser Vegetation Imaging Sensor (LVIS) or Scanning Laser Imaging of Canopy by Echo Recovery (SLICER), have successfully flown in preparation for future satellite-based lidar missions such as VCL or GLASS. Based on the state-of-the-art today, it is likely that by 2005 waveform-capture sensors will be in operation in the commercial sector. Such sensors will likely address niche commercial markets such as scientific research in forestry, topographic mapping beneath very dense canopy, and calibration/validation of global lidar data sets available from satellites such as VCL. They may also find a market in large area topographic mapping, such as statewide mapping efforts, when grid-spacing requirements do not demand the high-density capabilities of a high repetition rate time-of-flight sensor.

Laser Bathymetry
Laser bathymetry is a related discipline to laser altimetry, using a pulsed laser source to measure subsurface topography in coastal waters. Laser bathymetry has already been demonstrated as an effective operational tool in hydrography¹⁶ . To date, the commercialization of this technology has been limited due to many of the same barriers encountered in the terrestrial lidar mapping sector and the significantly higher capital costs of the bathymetry sensors, on the order of $5 – $10 million. Laser bathymetry also requires more sophisticated data processing and analysis than is generally used in topographic mapping. However, there are emerging markets for coastal zone mapping and near-shore bathymetry that may offer commercial potential for firms operating bathymetric lidars. Indeed, there is a strong synergy between laser bathymetry and topographic large footprint waveform-capture sensors. While terrestrial and bathymetric lidar sensors are currently distinct instruments addressing different mapping requirements, there is growing interest in the potential for hybrid sensors that combine the capabilities of both. A sensor capable of capturing the high-resolution topographic detail of a terrestrial mapping sensor with the water penetration and waveform-capture capabilities of a bathymetric lidar would be of considerable interest in the near-shore environment. Such a sensor could provide cost-effective data collection of both lower resolution bathymetric data and a higher resolution terrain model of the shoreline. The U.S. Navy’s latest generation of laser bathymeters, CHARTS – Compact Hydrographic Airborne Rapid Total Survey – is already under development by Optech, Inc. and will incorporate laser bathymetric soundings at 1 kHz with simultaneous topographic surveying at 10 kHz¹⁷ . The development of a commercial market for such a sensor will be driven by the economic value placed on mapping the coastal zone. However, given the high priority of research in this environmentally sensitive and economically important region, it’s likely that such hybrid sensors will be in commercial operation by 2005.

Platforms
The majority of commercial lidar mapping today is conducted from small fixed wing aircraft or helicopters. However there are other options for platforms that may become operational by 2005. Unmanned Aerial Vehicles (UAVs) offer an interesting option for mounting lidar sensors, either alone or as part of a suite of instruments. They are particularly attractive when long on-station surveillance is required or when operational conditions prohibit or restrict manned flight. Cleary, military reconnaissance and surveillance missions dominate this field but there is potential for niche markets in law enforcement. In addition to alternative airborne platforms, laser altimetry from spaceborne platforms is under active investigation. Within the next five years, lidar mapping from space will become a reality with the launch of NASA’s VCL and GLAS satellites. This generation of lidar satellites has been developed to address scientific objectives but there may be commercial potential for the data that will be generated from orbit, especially for forest canopy mapping at the regional scale or larger. More important from the commercial point-of-view, these satellite missions will also demonstrate the feasibility and cost-effectiveness of providing lidar data from space. Similar to the imagery market, where high resolution images from satellites such as IKONOS are available, satellite-based lidar remote sensing should act as a compliment to airborne laser altimetry, although in areas where lower resolution and accuracy are required over large areas they may supersede airborne platforms. Given the significant capital costs and lead time required to plan and execute a satellite-based mission, it is unlikely a commercial lidar satellite sector will develop in the next five years but it has the potential to develop by 2010.

Conclusion: Barriers to Further Adoption
As with any new remote sensing technology, early entrants and technology enthusiasts can provide the initial demand but development into a mainstream market does not automatically follow from early adoption. While developments during the past five years have been encouraging for advocates of laser altimetry as an operational remote sensing tool, the general positive mood evident today among many supporters of lidar mapping may be premature and misleading. Lidar mapping is still regarded with significant skepticism by many mainstream end users, in part based on early experiences that have been less than positive. As with many new technologies, early adopters have horror stories of poor data, failure to meet specifications, repeatedly missed schedules and hidden costs that have collectively served to increase the skepticism about lidar mapping’s capabilities and value. Cautious skepticism and a rigorous, peer-reviewed analysis are a necessary and appropriate counter-balance to the potentially biased optimism of the instrument vendors and data providers. Government agencies and professional associations such as ASPRS clearly need to play a leadership role in this area¹⁸ while all sides of the debate need to exercise reasonable, open-minded judgment if the potential of this exciting technology is to be fully-realized by the mainstream.

While its early successes – and failures – appear to be behind us, laser altimetry still needs to overcome additional barriers before becoming truly integrated into the mainstream remote sensing, photogrammetry, survey and mapping sectors. Barriers that must not be ignored if greater adoption is to be encouraged. Barriers to wider-spread adoption include the high capital costs to acquire or develop a sensor, a lack of published guidelines or standards for the professional practice of lidar mapping, the perceived threat to existing value chains posed by the new technology, disagreements and conflicting opinions about the achievable performance of the sensors, and a poor fit between the nature of lidar data and the existing mapping standards developed for older, more mature technologies. In looking forward over the next five years these barriers are clear challenges that all stakeholders in this technology must work together to overcome. But there are steps we can collectively take to ensure adoption of this valuable technology does not falter or reverse. These include:

1. Promote laser altimetry as a viable remote sensing tool with significant strengths, while openly acknowledging its limitations and weaknesses.

2. Educate the end users. An educated, knowledgeable and engaged end user community will significantly strengthen the reputation of laser altimetry by requiring data providers to adhere to professional practices and procedures while routinely providing quality data on time and on budget.

3. Create a community of practitioners, users and stakeholders. The ultimate success of laser altimetry as a remote sensing tool depends on the ability of the tool to rise above the interests of individual stakeholders and be embraced by a larger community of remote sensing and mapping professionals whose goal is the widest possible adoption of this technology.

Highway overpass in North Carolina in plan view captured using Airborne 1’s Optech 1225	Draped imagery on lidar DEM from Interstate #5 near of Los Angles, California.

Remote sensing technologies are valuable contributors to our on-going efforts to study, understand and mange our planet and our environment. Laser altimetry is an extremely powerful addition to this remote sensing toolbox that provides us with an exciting new way of looking at the world. Given the significant value that laser altimetry can contribute to our knowledge and our decision–making processes, decisions that range from mapping the surfaces of distant planets to estimating the global biomass to protecting our environmentally-sensitive coastlines to assisting in making informed decisions on commercial development, it is important that all stakeholders in this technology step forward to address the challenges ahead and help continue the transition from science to commercial lidar mapping.

References
1. The terms laser altimetry and lidar will be used interchangeable in this article with the additional understanding that in general we are referring to small footprint, time-of-flight topographic lidars not waveform-capture or bathymetric lidars. The term small is subjective but is generally taken to mean a footprint on the ground no larger than 1 m.

2. See http://aol.wff.nasa.gov/ for details and publication references related to the AOL sensor.

3. Krabill et. al., 1984. Airborne laser topographic mapping results. Photogrammetric Engineering and Remote Sensing, 50(6), pp. 685-694.

4. Garvin, J.B. et. al., 1998. Observations of the Earth’s topography from the Shuttle Laser Altimeter (SLA); laser-pulse echo-recovery measurements of terrestrial surfaces. Phys. Chem. Earth 23 (9-10), 1053-1068.

5. Dubayah, R., et al., 1999. The Earth as Never Seen Before: VCL and the Lidar Revolution in Land Surface Characterization. In Proc. Amer. Geo. Phys. Union, 1999 Fall Meeting.

6. Schutz, B. E. and J.H. Zwally, 1993. Geosciences Laser Altimeter System (GLAS): A Spaceborne Laser Altimeter for Ice Sheet Mass Balance Applications, Eos Trans. AGU, 74, 181.

7. Smith, D.E., et al., 1998. Topography of the northern hemisphere of Mars from the Mars Orbiter Laser Altimeter. Science 279 (5357), 1686-1692.

8. Blair, J.B., et al., 1999. The Laser Vegetation Imaging Sensor: a medium-altitude, digitization only, airborne laser altimeter for mapping vegetation and topography. ISPRS Journal of Photogrammetry and Remote Sensing, 54, 115-122.

9. M. A. Hofton, et. al., 2001 Validation of Vegetation Canopy Lidar sub-canopy topography measurements for a dense tropical forest. To be published in Journal of Geodynamics.

10. Wehr, A. and Lohr, U., 1999. Airborne laser scanning - an introduction and overview. ISPRS Journal of Photogrammetry and Remote Sensing, 54(2/3), 68-82.

11. Baltsavias, E.P., 1999a. Airborne laser scanning: basic relations and formulas. ISPRS Journal of Photogrammetry and Remote Sensing, 54(2/3), 199-214.

12. Blair, J.B., et al., 1994. Optimization of an airborne laser altimeter for remote sensing of vegetation and tree canopies. IGARSS-94 Surface and Atmospheric Remote Sensing: Technologies, Data Analysis and Interpretation. IEEE, Pasadena, CA.

13. Flood, M., 2001. Commercial Lidar Technology: The Next Five Years. in Proc. ASPRS Conference 2001 (St. Louis).

14. For details see http://duff.geology.washington.edu/data/raster/lidar/index.htm.

15. For details see http://www.ncfloodmaps.com/.

16. Irish, J.L. and Lillycrop, W.J., 1999. Scanning laser mapping of the coastal zone: the SHOALS system. ISPRS Journal of Photogrammetry and Remote Sensing 54, 123-129.

17. See Airborne Lidar Bathymetry Executive Notes [Vol. 2(#1), May 2001] available from the Joint Airborne Lidar Bathymetry Technical Center of Expertise.

18. The ASPRS Lidar Subcommitte of the Photogrammtric Applications Division is addressing many of these areas. For details please contact ASPRS or visit http://www.asprs.org . Volunteers are welcome. In addition, ASPRS has published a new book, entitled Digital Elevation Model Technolgies and Applications: The DEM Users Manual, that is now available from ASPRS at http://www.asprs.org. The book is aimed at users of the technology to help them understand how and when the technology can best be used.

Martin Flood is chief technical officer for Airborne 1 Corporation, a lidar data provider based in in Los Angeles, California. Prior to joining Airborne 1 he was Program Manager and Terrestrial Survey Team Leader at Optech, Inc. (Toronto, Canada) and was founder/publisher of the industry reference web site airbornelasermapping.com. He is an ASPRS member and sits on the LiDAR Subcommittee of the ASPRS PAD Committee.

Airborne 1 Corporation
5777 West Century Blvd. #725
Los Angeles, California 90045
flood@airborne1.com

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