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|Federal Highway Administration > Publications > Public Roads > Vol. 65 · No. 2 > Low-Altitude Laser Surveys Provide Flexibility and Savings|
Low-Altitude Laser Surveys Provide Flexibility and Savings
by Lisa Crye
The Federal Highway Administration's Central Federal Lands Highway Division (CFLHD) Survey and Right-of-Way Team recently tackled two challenging road surveys with low-altitude LiDAR terrain measurements made from a helicopter. LiDAR, which stands for light detection and ranging, has been used frequently to map and survey from airplanes and satellites.1,2 However, lower altitude applications have been less common.
Both surveys were needed to plan upgrades to existing gravel roads in environmentally sensitive areas. In both cases, a helicopter equipped with a low-power laser collected in a few hours the complete range of required information that would have taken several weeks for a ground survey crew to collect.
CFLHD used the LiDAR surveys for Guanella Pass, a Colorado scenic byway, and for an access road through the Bear River National Wildlife Refuge in Utah.
The Guanella Pass road is paved in some places and gravel in others. It is plagued by erosion, sedimentation, and maintenance problems. The proposals for repair and upgrade in the draft environmental impact statement have been controversial with environmentalists. Ultimately, CFLHD has settled on a combination of reconstruction and rehabilitation for the road. The plan includes adding some retaining and fill walls to stabilize eroding cut slopes and rebuilding ditches and culverts to address drainage problems, while retaining the rustic character of the road.
The road, which is 37 kilometers (23 miles) long, has elevations ranging from 2,600 meters (8,500 feet) beginning in Georgetown, rising to 3,500 meters (11,500 feet) at the pass, and dropping to 2,750 meters (9,000 feet) in the town of Grant. In addition, it is heavily wooded, which means that traditional aerial mapping techniques cannot "see" the ground surface. The LiDAR technology, however, can penetrate a forest canopy the way sunlight does to obtain ground measurements. The design solutions for this sensitive area require a higher level of precision than conventional photogrammetric techniques (making surveys and maps using aerial photographs) can provide and more extensive coverage than terrestrial survey techniques can provide.
In the case of Bear River, the problem was neither the ruggedness of the country nor the heavy tree cover. It was a tight timetable. The Bear River National Wildlife Refuge lies northeast of the Great Salt Lake and just west of Brigham City, Utah. It is a marsh and a migratory bird refuge with a 19-kilometer (12-mile) loop of dike roads that were damaged by flooding.
"We needed data as the snow melted but before the water rose," said Alan Blair, CFLHD survey team leader. This made planning for a survey crew difficult because so much depended on timing and weather.
For both surveys, CFLHD wanted an accuracy of 15 centimeters (6 inches) horizontally and 10 centimeters (4 inches) vertically. To draw cross-sections and produce designs, they wanted a digital terrain model (DTM) and an ortho photograph (taken from directly above and perpendicular to the ground) that had been geo-referenced so that measurements would be correct relative to the ground.
CFLHD preferred a survey method that would not require extensive ground crews or demand a great deal of access. Both projects were environmentally sensitive, and Bear River had a timetable dependent on snow melt and water levels. Accurate aerial mapping using traditional photogrammetric techniques requires multiple identifiable control points visible in the photographs. When there are no such identifiable points — called photo IDs, which are planimetric features — in the landscape, which is often the case in remote areas, ground crews must place them.
Surveyors can speed the process of coordinating the control points necessary for an aerial survey by using the Global Positioning System (GPS). The U.S. Department of Defense developed GPS to provide 24-hour, all-weather navigation for military forces.3 The system uses 28 orbiting satellites that send coded signals that can be processed in a GPS receiver to compute position, velocity, and time.3 GPS has caused a revolution in land surveying because a line of sight along the ground is no longer necessary to determine a precise position.
The CFLHD Survey and Right-of-Way Team knew about the use of LiDAR in conjunction with GPS to perform surveys with fixed-wing aircraft from high altitudes, but these would not have provided the accuracy they needed. To design the walls and other critical features along the Guanella Pass byway, survey team leader Alan Blair wanted "an inexpensive way to ground truth our aerial survey."
At a conference and in subsequent discussions with representatives of John Chance Land Surveys Inc., Blair learned that the whole route could be surveyed in a day and that the survey could also provide a video that could help participants at public meetings visualize the effect of the proposed design.
LiDAR and FLI-MAP®
During the 1990s, John E. Chance and Associates Inc. of Lafayette, La., a division of Fugro, developed a system called FLI-MAP®, which stands for fast laser imaging, mapping, and profiling.
The system integrates LiDAR, GPS inertial sensors, and S-VHS video to gather geographic information that can be delivered in a number of software formats. The LiDAR unit is mounted to a helicopter, and the sensors scan the ground at 10 to 20 points per square meter. The FLI-MAP system has two GPS antennas mounted to the system pod, located underneath the helicopter. The GPS antennas are mounted on booms that extrude from under the helicopter to the left and right. GPS is used to give position and time to the system, and the IMU (inertial sensor) is used to correct for heading, pitch, and roll as the system flies over the project corridor. The navigational information collected by the GPS receivers and the inertial sensor combines with data collected from GPS base stations on the ground to give an accurate position of the helicopter every half second. Both surveys used four GPS base stations at 16- to 24-kilometer (10- to 15-mile) intervals.
At the same time that LiDAR and GPS are collecting data on terrain and position, two S-VHS video cameras collect high-resolution, time-stamped video in front of and below the helicopter. The two cameras — one pointed at a 45-degree angle forward and the other pointed straight down — serve different purposes, said Blaine Thibodeaux, the Chance representative who worked with CFLHD on the Guanella Pass and Bear River surveys. The forward-pointing camera, which produces images one might see from the helicopter cockpit, is used for reference. The images from the 90-degree camera are used to produce video imagery that can be used as a visual aid along with the LiDAR data. The two can be merged to produce a geo-referenced image (mosaic digital image) capable of being imported into most computer-aided design and drafting (CADD) packages.
The helicopter platform allows flexibility as well as a low altitude — usually 50 to 100 meters (164 to 368 feet. Both Guanella Pass and Bear River were flown at the 50-meter level.
Surveys and Deliverables
CFLHD asked for an 80-meter (260-foot) corridor of survey data on the Guanella Pass road. This meant, Blair said, that the helicopter could basically "fly down the middle of the road," using it for a reference. It turned out not to be quite that easy, however, in the mountainous terrain. A helicopter "just about maxes out at 11,500 feet, depending on temperature," and in addition, wind was a problem. The helicopter survey crew usually includes the pilot, an equipment operator, and a navigator or client observer.
Unlike Guanella Pass, Bear River had elevations of 1,280 to 1,284 meters (4,200 to 4,215 feet) and no forest canopy. In this case, CFLHD wanted a wider corridor, so the helicopter had to make five passes to measure all the terrain, said Blair.
To do this, the surveyors built flight lines in the air and navigated flight routes following GPS instruments, explained Thibodeaux. They do this when there is no right of way to follow or, as in the case of Bear River, when "the minute you get off the road, you can't tell where the right of way is located."
For both surveys, Chance delivered an ASCII-formatted DTM consisting of three-dimensional coordinates (spatial points) and a geo-referenced color digital image, called a "mosaic" because it is prepared from an assemblage of images. The mosaic is two-dimensional, but it is what cartographers translate into a 3-D drafted map using the XYZ coordinate files. The data was delivered on digital video disk (DVD).
Federal Highway Administration road designers process this information to establish triangular surfaces connecting the spatial points, which adds the third dimension to the two-dimensional image. The resulting data from the surveys gave designers the information they needed to draw cross sections, calculate drainage ditches, changes in road elevation, and amounts of gravel or earth needed to upgrade the roads. It will also give construction crews a better representation of the area that they are working in. In addition, it was faster and saved "about two-thirds of what a survey crew on the ground would have cost."
The Right Tool Provides Flexibility
Both Thibodeaux and Blair stressed that LiDAR mapping is not suitable for every project.
"It's not going to replace traditional survey techniques," Blair said. "We will use this technique when we can settle on a particular corridor. If we aren't sure where we're going to put a road, we'll use traditional aerial photogrammetry."
Laser mapping can be particularly helpful when permission is not granted for survey access, terrain is too rough, or the area is heavily wooded. In addition, it is environmentally friendly. Even the GPS base stations do not have to be placed directly on the survey corridor. Another major benefit of this method is its reduced processing time to produce the DTM — usually about two weeks after the survey flight. This kind of turnaround can save months of staff time and expense.
While LiDAR may not be the right tool for every survey, its flexibility and accuracy provide options that can save time and staff expense on the right projects.
"We're always looking for better, less expensive ways to get our work done," said Blair. "We selected projects where we thought it would be effective, and we'll use it again under the right circumstances."
1. W. Krabill. "Greenland Ice Sheet: High Elevation Balance and Peripheral Thinning." Science, Vol. 289, No. 5478, The American Association for the Advancement of Science, July 21, 2000, pp. 428-430.
2. W. Krabill and C. Martin. "Aircraft Positioning Using Global Positioning System Carrier Phase Data." Navigation, Vol. 34, No. 1, Spring 1987, pp. 1,211-1,222.
3. Peter H. Dana. "Global Positioning System Overview," The Geographer's Craft Project, University of Colorado at Boulder, 1999. http://www.colorado.edu/geography/gcraft/notes/gps/gps.html
Lisa Crye is a freelance writer and editor. She has written for publications as varied as Science and The Arlington Historical Journal and edits newsletters and a research journal. Her work has focused on business, environmental, medical, and governmental contract issues.
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