Airborne: Seeing in 3D Without the Glasses
By Mary Jo Wagner

It's 4:00 p.m. on an overcast Thursday afternoon. Jim gets a call from management. They need a Digital Terrain Model (DTM) derived from an airborne survey of a particular area by tomorrow morning. Jim rushes to the nearest surveying company. Within two hours, the aircraft is surveying the area requested and records one hour of data. By that evening data processing has begun and Jim finds the DTM on his desk the next morning.
      Is this an airborne remote sensing system of the future? No, this is the Airborne Laser Terrain Mapping (ALTM) 1020 system and it's here now.
      The above example could be considered extreme, but apart from the urgent circumstances, the rest is right on target. The ALTM system can operate and record data at night. It can record data on overcast days. It can be installed in about two hours. Data processing can be done overnight. And this system guarantees 15 cm absolute terrain height accuracy.
      No small feat for a company that started out as a one-man show. As a professor at York University, Dr. Allan Carswell recognized a number of laser technologies that were being researched but were not commercially available. Carswell founded Optech Inc. in 1974 to try and change that. Since its inception, this Toronto-based company has researched and developed over 10 advanced laser radar (LIDAR) systems and supplied them to a worldwide market. One of their products, the SHOALS/HAWKEYE airborne LIDAR bathymeter system, recently brought them the Canada Award for Business Excellence in innovation.
      Optech's first LIDAR products were ground-based systems, but since then they have extended their manufacturing line to include airborne remote sensing systems as well. Which brings us to the latest and most innovative addition to Optech's reputable, high-quality line of products. The ALTM 1020.
      The ALTM was designed to carry out just what its name implies: to map terrain - regardless of what type - using a laser sensor. This laser measures the distance between the aircraft and the ground and typically fires 2,000 pulses per second, recording 2,000 data sets a second.
      But that's just the laser sensor. Without all of the other components of the system, it wouldn't work. There is the 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 scanner which directs the laser pulses across the flight path and collects the reflected light. The time of each shot and angle of the scanner are also recorded. And there is the specially designed software which pulls all of this information into a comprehensible form.
      All of the system parameters can be changed in flight. The repetition rate of the laser can be altered. The laser can be programmed to either record the first object that it senses in the flight path or the last. The swath width can be set anywhere from 0 m to 730 m, depending on altitude. The accuracy will still be the same no matter what. This flexibility allows more freedom to the end user and can make data acquisition faster and less costly, making this system a viable alternative to traditional mapping methods and competing technologies.
      According to Mr. Don Carswell, ALTM product manager at Optech, this kind of system is especially useful in forested regions. "The ALTM has the capability to penetrate tree canopy," he said in a telephone interview. "Aerial photos are limited in their penetration in dense trees. We've done demonstrations in the Pacific Northwest of the States with Fir and Hemlock-mixed forest. In high summer we penetrated 40 percent of our shots to ground. We actually found ravines under that canopy which were not visible from the air photos. We're actually able to see through the trees." An ability, he adds, that doesn't currently exist with any other available system.
      That could be why the ALTM is a hot item right now and has been since 1993 when the first system was delivered to clients in Germany.
      But the ALTM probably may not have come about had it not been for two German doctorate students at the University of Stuttgart, who as a matter of fact, were the clients in Germany. Dr. Peter Friess and Dr. Joachim Lindenberger were using an airborne laser profiling system, basically a laser altimeter from Optech to conduct research. They noticed from the data that they were able to penetrate some foliage on the ground and tree canopies, enabling them to predict terrain surfaces. Friess and Lindenberger saw an opportunity and they seized it.
      They contacted Optech to discuss the possibility of manufacturing a more sophisticated laser system with a scanning laser instead of a profiling one. They prepared a list of requirements to meet their goals. Friess and Lindenberger wanted, for example, to penetrate tree canopies at the highest possible percentage of laser shots. Optech matched their requirements with the birth of the ALTM 1020.
      While Optech was busy creating the new system, Friess and Lindenberger were busy designing and producing the software for the system. They also went into business for themselves, establishing their surveying company TOPScan in 1992. Thus TOPScan has become a corporate partner to Optech. Optech supplies the hardware - the ALTM - and TOPScan provides the software. A very important element indeed for without the post-processing software, Optech could not guarantee the high terrain accuracy of 15 cm.
      In essence, the ALTM was designed by end users. A rather backward approach as most manufacturers create products and then ask the end user what they think. By getting the input directly from users first, Optech has created a custom-made product and a market at the same time.
      Since the client will dictate the kind of survey performed, not every flight will be the same. But a typical survey would go something like this. The only information necessary to begin planning the flight are the coordinates of the target area. This system does not require reference locations or ground control points. Once the coordinates are known, the GPS ground-based receiver is set-up at a known place in the survey area. The operator sets the appropriate ALTM and aircraft parameters and they are ready for take-off.
      While in flight, the operator monitors the collection of data and adjusts any parameters necessary. The laser starts recording data and is fired into a single axis scanner. The scanner scans across the aircraft's flight path, in the direction of its wing tips, tracing a saw-tooth pattern on the ground. Whether the laser has been programmed to record the first pulse or last pulse will determine which objects are sensed.
      Once on the ground, the data tapes are transfered to the post-processing system. The ALTM 1020 GBPP software initializes a survey database and the flight tapes and GPS data are read into it to check for completeness. The airborne and ground-based GPS data are merged to obtain the flight path and the measurements from the IRS are computed and referenced to a local coordinate system. Each ground point is automatically classified as ground reflection or vegetation or buildings. A DTM can then be derived from the laser points classified as ground reflections. Once the DTM or map is created, no field surveys are required to verify the accuracy.
      Optech claims the accuracy of the system is independent of altitude. Therefore, whether an aircraft is flying at the maximum altitude of 1000 m (1 km) or the minimum height of 300 m, the elevation accuracy will not falter. And that precision is warranted whether the survey area is a forest, a coastal zone or a sand dune.
      How the ALTM is successful over forested regions is because the laser uses the gaps in between the trees to penetrate the ground. Carswell explains it this way. "In a dense forest, if you look up at the sky, you'll typically find some openings in the tree canopy and you can see the sky from the ground. As long as there is that dappled light on the ground when you fly over that forest, some of the laser light should penetrate to the forest floor."
      Thus deriving forest inventories is possible through this method. By programming the laser to record the first pulse on the initial flight path, the sensor will record the first object it hits: the upper branches of the canopy. Then by switching to last pulse on the same flight path, the laser will sense the ground. A client will end up with two independent digital records of the same thing. One shows the trees and one doesn't. A simple volumetric measurement will calculate the tree volume.
      Coastal zones which have been traditionally a difficult area to survey with aerial photography are another region where the ALTM excels. In fact, Friess and colleagues have been out proving just that in the Netherlands.
      A government agency contacted them because they are interested in monitoring the changes in the tides from year to year. Surveys of years past have proven too costly so they wanted to try something new.
      "Last year they did the survey with conventional surveying means," said Friess. "Now they've tested the laser scanning because it's faster and you can cover larger areas in a shorter time. And for that reason, it's also more cost efficient." Carswell adds that the ALTM system can cover two acres a second.
      Friess and his group will be supplying the agency with a DTM of the coastal area of interest.
      In an urban setting, the system can identify faces of buildings and the rooms inside. It can identify curbs, making it possible to tell where the sidewalk is and where the road is.
      The ALTM can also be used for planning pipelines and inaccessible terrain. Once the piplines are laid, they can be monitored with the system because it can identify the pipelines and help predict possible rupture due to ground heating. The system can identify mountain sides swelling if their movement on the ground is just a few centimeters.
      But Carswell feels the real market area for the ALTM is topographic mapping. "We can map at any scale," he said. "We anticipate that most of our clients need large scale maps because this is ideal for that application. Small scale maps are readily available but the data for large scale maps is very poor so that's our target market."
      But airborne remote sensing has come under attack before due to its high cost and its vulnerability to air turbulance. Carswell is not worried about that and is prepared for it. They estimate the cost of this sytem is running at a much lower cost per area than stereo photogrammetry. The data acquisition cost is much lower due in large part to the fact that none of the labor-intensive work on the ground is necessary.
      They have also taken care of handling data error due to turbulance. Using sidelap allows for turbulance and prevents gaps in the data on the ground. This ensures a blanket coverage of the area of interest. If the aircraft is rolling, with this system they can measure its current attitude to better than .05 degrees, giving about a one-quarter meter resolution to where the ground impact is, despite air roughness.
      "I think we have a terrific technology," commented Carswell. "And everyone that I talk to about it is simply amazed at its capability."
      So it seems that Optech has really done its homework this time. And with new products in development such as an automatic power line identifier and mapper, or an airborne LIDAR system for detecting oil spills and measuring their thickness, they may continue to make cases like Jim's a reality.

About the Author:
Mary Jo Wagner is a freelance writer/editor who specializes in GIS and Remote Sensing. She can be reached at 715-235-7422 or through e-mail: [email protected]

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