| AIRBORNE: Terrain Mapping
The use of airborne laser scanning technology offers reduced processing times and data collectiion expenses for a number of applications.
By Martin Flood, Bill Gutelius and Matt Orr In the airborne remote sensing industry no other adage is more appropriate than "time is money." Because spatial data customers (everyone from GIS operators to civil engineers) require faster turn-around times and more accurate data, aerial operators look to more advanced technology to reduce processing times and data collection expense.
Emerging as one of the leaders in airborne laser scanning technology, Optech (North York, Ontario, Canada) specializes in laser radar applications, from ground-based measurements of atmospheric parameters to the airborne measurement of water depths. In the 1980s its airborne lidars produced the world's first nautical charts based on airborne lidar data, and in the early 1990s its ground-based differential absorption lidar systems were installed at several sites as part of the global effort to monitor stratospheric ozone. While Optech's first lidar products were ground-based systems, they have now expanded their product line to include airborne remote sensing systems as well.
The latest and most innovative addition to Optech's high quality line of products is the Airborne Laser Terrain Mapper (ALTM). The ALTM is designed to carry out just what its name implies; to map terrain - regardless of type and complexity - from the air using a laser sensor. The laser measures the distance between the aircraft and the ground at rates up to 5,000 pulses per second, recording 5,000 individual data sets per second. In addition to the laser there are GPS airborne and ground receivers which record the aircraft's position.
There is the Inertial Reference System (IRS) which reports the plane's roll, pitch and heading. There is the high accuracy scanner which directs the laser pulses across the flight path and collects the reflected light from the ground. The time of each laser shot is recorded by the airborne computer along with range, scanner angle, GPS position and IRS attitude data for every laser shot. The data is downloaded after the flight and rapidly processed by the specially designed post-processing software which combines all of the information to produce accurate, absolutely referenced XYZ positions for every data point. Another advantage of collecting elevation data with an ALTM is that by applying appropriate classification algorithms during post-processing, it is possible to classify each data point as a ground reflection or a reflection from vegetation or a building. Once classified, removal of the over-lying vegetation layer is straight forward and allows the generation of a DTM of the ground beneath the tree canopy. Coastal Zone Terrain Measurement
The ability to create DTMs under adverse conditions is being widely utilized in a growing number of industries, where previously accurate elevation data were hard to obtain.
Historically, coastal zones have presented a formidable challenge to mapping scientists. Trying to gauge the amount of change along a shoreline is difficult due to the nature of the coastal environment. Photogrammetry is difficult to carry out when there is limited contrast as is found along beaches and dunes and also where vegetation proves too thick for estimation of the ground surface below the canopy. Airborne lasers are capable of penetrating much of the vegetation that grows along coastal areas and the laser reflects extremely well off sandy/rocky materials. This allows for determination of even the smallest change in relief. Standard land surveys of sand bars and barrier beaches are often difficult or impossible due to their remoteness. Many coastal morphological features are difficult to survey due to fluctuations in tides and also the rapid shifts in the sand that makes up these features.
Being able to quickly assess beach erosion and accretion zones is essential to governmental agencies who have the tough task of managing the highly complex and dynamic coastal environments, many of which are becoming densely populated. It is this ever increasing population pressure that makes mapping of coastal zones such a critical component of coastal zone management. Florida
As part of a feasibility test conducted on behalf of the University of Florida, Florida Department of Transportation (DOT) and the Department of Environmental Protection (DEP) in the Florida Panhandle during October 1996, the ALTM was used to map 350 km of coastline. The objective of the project is to assess coastal change due to the effects of hurricane Opal several years ago. Traditional methods (land survey, photogrammetry) had been previously employed to measure the changes in shoreline features since Opal made landfall. However, the researchers require a faster, more accurate method to determine damage levels and coastal alterations due to storm surges and other severe weather events, as well as longterm, continuous feature changes. According to an official from the Florida DEP, the area that was used for the study took a team of surveyors six months, using kinematic GPS units, to measure profiles of the beach and dunes at 1,000 foot intervals along the beach. Each profile had a few hundred points beginning at the water, continuing to the edge of dune/vegetation interface region. That same area (350 km of coastline, approx. 230km2) was flown in a little over two hours using an ALTM which provided continuous point coverage from beginning to end. The total number of elevation points, individually geo-referenced, was approximately 27 million. This translates to a point density of greater than 100,000 /km2. (See Figure 1 for an example of a DTM generated from the Florida feasibility test.) United Kingdom
In the United Kingdom there is also great concern for coastal zones. With large fluctuations in tides, many estuaries and tidal areas are subject to massive amounts of silt movement. Erosion, deposition and floodplain vulnerability are all critical issues to the United Kingdom Environment Agency (UKEA). The UKEA is tasked with the very difficult job of monitoring the transport and volumetric changes in silt flats and sand bars. In November and December 1996, Optech participated in an evaluation of airborne laser terrain mapping as a potential method for coastal and riparian zone topographic measurement in the U.K.
Obviously, it is extremely difficult to gain pedestrian access to soft silt flats which also do not lend themselves to being easily discerned in aerial photos, even to a seasoned photogrammetrist's eye. Whereas, the ALTM can achieve measurable success in mapping the elevation and location changes in silt flats and sand bars. The results from the surveys over southeast England show that along with enhanced point density, more areas previously difficult to survey or even unmeasurable, can now be assessed with a high degree of absolute, not just relative accuracy. Forestry
The timber and forestry industries have realized time and cost savings in employing the services of companies operating the ALTM. Determination of the terrain elevation below tree canopies has always proved difficult for photogrammetrists. The problem with dense vegetation is that the terrain below is not always clearly visible, if at all. With an airborne laser, at least some of the shots will reach ground level. Experience has shown the rate of penetration of the ALTM is often better than 30 percent in thick coniferous, deciduous and mixed forests. During survey operations, all of the system parameters can be adjusted by the operator. The repetition rate of the laser and the frequency of the scanner can be altered to suit the particular survey and changing terrain. The laser can be programmed to operate in either first pulse or last pulse mode; effectively detecting either the first target in its path or the last. This can be particularly useful when trying to map targets above the ground, such as tree canopies or powerlines. One of Optech's first ALTM customers, Airborne Laser Mapping of Seattle, Wash., routinely maps heavily forested areas of the Pacific Northwest and uses the unique features of the ALTM to generate accurate DTMs of the forest floor despite the heavy tree cover. (See Figure 2.) ALM serves a variety of public and private sector clients. The company provides laser based mapping services for forestry, engineering, local government and other applications in the Western United States. ALM maintains offices in Seattle, Wash. and Phoenix, Ariz. Powerline
Around the world, many thousands of miles of powerline require assessment of critical points, known as "danger-tree" locations, where the powerlines are encroached upon by rapidly growing vegetation. The fastest and most economical way to accomplish this is by flying an airborne laser scanner down the powerline corridor. The information collected by the system includes the elevation of the terrain below the wires, the structure of the tower and its elevation, the elevation of trees and other vegetation under and around the powerlines and tower, and the proximity of vegetation to powerlines and tower. Conclusion
As computers become faster and data manipulation and storage capabilities increase, sensors such as the ALTM will provide all the digital elevation data that a user could require. Despite delivering systems around the world and servicing the overwhelming demand for demonstration surveys by potential customers, Optech is not content to rest on its laurels. They are continually looking at ways to improve the performance of the system based on feedback from their current customers. Improved technical performance areas for the ALTM including higher operating altitudes, improved laser and scanner performance and simultaneous measurement of ranges to multiple targets for each laser pulse. These and other key enhancements have already been identified as highly desirable features and are currently under development by Optech's research and development teams. About the Authors:
Martin Flood is team leader, Terrestrial Survey Team at Optech in Toronto, Canada. He can be reached by e-mail at [email protected]. Bill Gutelius is an ALTM product specialist at Optech. He can be reached by e-mail at [email protected]. Matt Orr is a manager at Airborne Laser Mapping in Seattle, Wash. He can be reached by e-mail at [email protected]. Back |
