LIDAR For Hire
Intense is the word U.S. Forest Service geologist Jim Baichtal uses to describe what it’s like to map Kosciusko and Tuxekan Islands in southeast Alaska. But, that’s an understatement according to others who consider the islands a surveyor’s nightmare.
Accessible only by boat or floatplane, the two islands lie about 30 miles offshore from Prince of Wales Island in the strip of Alaska that hugs British Columbia. Getting to the islands isn’t nearly as hard as getting around on them. Slopes of 45 degrees are common, and the terrain is pocked with 80- to 120-foot-deep depressions and sinkholes. These features, all typical of karst topography, make travel difficult and dangerous.
The vegetation doesn’t help much, either. Heavy rains cultivate a lush undergrowth that obscures the forest floor and hides cave openings in a single growing season. Towering above is a dense mixture of Sitka Spruce, Western Hemlock and Alaska Yellow Cedar that make it nearly impossible to accurately map the ground surface from air or space.
“We would spend days and days on the ground trying to map the features as best we could, but our inventory results were often difficult to accurately display,” said Baichtal, who works in the U.S. Forest Service’s Thorne Bay Ranger District Office on Prince of Wales Island.
Surveying the sparsely populated islands, however, is a necessity because their softwoods are sought by timber companies, working in the area since just after World War II. Since Kosciusko and Tuxekan Islands reside inside the Tongass National Forest, they are subject to the requirements of the Federal Cave Resource Protection Act of 1988 as interpreted by the Forest Service. This intensifies the need for accurate mapping on the islands.
In general terms, the Act requires the government to protect any caves found on federal lands from activities that might damage them, including logging falls.
“As soon as we saw the islands, they reminded us of a similar mapping project near Seattle where dense tree canopies made it impossible to map surface features from the air,” said Rik Langendoen, a URS associate geologist. “The solution to that problem was LIDAR (light detection and ranging) technology, and we believed it would be the answer on Kosciusko and Tuxekan as well.”
The Forest Service was skeptical but wanted to give the technology a try. URS contracted EagleScan Remote Sensing based in Boulder, Colo., a division of 3Di LLC in Easton, Md. Under the contract, EagleScan flew its LIDAR-based Digital Airborne Topographical Imaging System (DATIS) over the islands to map under the forest canopy in the proposed harvest sites.
The skeptics quickly learned why LIDAR has become the fastest growing remote sensing technology in use today.
“We were quite shocked on Tuxekan, for instance, where the LIDAR showed 100 percent of the karst features we had identified in the pilot site and another 15 to 20 percent we hadn’t seen,” said Baichtal. “And the system was accurate—the features were right where they were supposed to be.”
The DATIS system proved its usefulness for mapping far more than just karst topography, Langendoen adds. The ability to reveal subtle topographic features is merely an example of how LIDAR technology has advanced to become applicable in many types of forest mapping projects.
“It can map any terrain hidden under a dense forest canopy,” he said. “In our environmental assessment, we performed extremely accurate slope analysis and bedrock geology mapping with the data.”
Technology Made for Elevation MappingIn 1994, EagleScan became one of the first commercial mapping companies to use LIDAR technology. The firm developed DATIS and all related software using customized methodologies. Mapping success in flood plains, utility corridors and forest inventories quickly followed, but projects in dense temperate rainforests had seldom been attempted.
LIDAR technology has become extremely popular in the last few years as a cost-effective means of collecting the digital elevation models (DEMs) required for orthorectification of aerial photographs and satellite images. LIDAR has proven itself as an accurate and practical alternative to the conventional method of generating DEMs.
In non-vegetated environments, DATIS has produced DEM data with a 0.10 RSME (root square mean error) vertical accuracy. However, in densely forested areas such as southeast Alaska, this cannot always be attained because of the influence of low ground vegetation beneath the canopy. Despite this, DATIS achieved the vertical accuracy required to meet the Forest Service’s desire to produce maps of the islands with 10-foot vertical interval contours.
As a mapping technology, LIDAR seems custom made for surveying ground surface elevation because it uses a laser to directly measure the Z values for up to 220,000 points per square mile. In an airborne configuration, the laser scans across the flight path to cover a half-mile swath in a single pass, mapping a large area in a short time.
The DATIS LIDAR operates at a 4 kHz frequency pulse rate, enabling it to emit and measure 4,000 light pulses per second. These pulses strike the ground and reflect back to the sensor, which determines the exact distance between the aircraft and the surface by timing the return. If the position of the system and the angle at which the pulse was emitted are known, the elevation of the ground point where the laser pulse hits can easily be calculated.
EagleScan integrated its LIDAR sensor with a GPS receiver and Inertial Measurement Unit (IMU) in its aircraft. The IMU provides the precise aircraft and sensor attitude data (pitch, roll and yaw) needed to determine the elevation of each point. The GPS provides the x, y and z position of the aircraft and sensor. In addition to the onboard GPS, a second geodetic quality receiver is placed on a high-accuracy ground bench mark in the project area to differentially correct the aircraft GPS data.
“There is no need to mark multiple targets on the surface,” said Sean Morgan, EagleScan’s processing manager. “In the Alaska project, we worked off of one National Geodetic Survey point at the airport on Prince of Wales Island.”
DATIS includes a digital camera, which collects panchromatic (black & white) imagery that is optically aligned to the LIDAR sensor. The LIDAR DEM can then be used to orthorectify the digital images to create extremely high-resolution digital orthoimages. Because only digital processing is involved, output products can be delivered within days of acquisition.
The EagleScan LIDAR can be operated in two different modes whereby the elevation measurements are made either with the first or last return signal. In topographic mapping over vegetated areas, LIDAR signals do not return to the aircraft at the same rate. Those that return first have usually been reflected from the tree canopy, while the last to return penetrated the vegetation and reflected off the ground surface.
This dual-mode capability was extremely valuable in mapping the surface topography on Kosciusko and Tuxekan Islands.
Mapping the TongassAt 17 million acres, the Tongass National Forest is the largest forest managed by the U.S. Forest Service. Roughly 10 percent of its surface area is underlain by limestone rock formations. This bedrock is composed of calcium carbonate, a substance extremely soluble in water. As a result, limestone bedrock in high-precipitation areas is transformed into a landscape of sinkholes, caves and topographic depressions. These features are the signature characteristics of karst topography.
When plans are proposed to sell timber in a national forest, the Forest Service routinely maps the target areas as part of an environmental impact assessment. The maps are also used to calculate the volume of wood in the areas so minimum prices can be set and it can be determined where to build roads. Passage of the Cave Protection Act introduced the need for geologic interpretation of topographic maps in forests underlain by karst.
“Our objective with the maps is to determine where it’s reasonable to harvest,” said Dave Every, an associate ecologist with URS Corporation. “By identifying significant features such as caves, we can establish buffer zones around them where no timber harvesting will occur.”
URS coordinated with EagleScan and the Forest Service to perform the LIDAR mapping in March 2000. March was chosen because foliage would be at a minimum.
EagleScan maintains its own aircraft at an airport near Boulder, Colo., but prefers to rent a suitable airplane for projects outside the lower 48 states because it’s less expensive than ferrying the plane. The DATIS equipment weighs about 350 pounds but is portable enough to allow for shipment. Technicians fit the system into a single-engine aircraft with a standard 18-inch camera hole in the belly.
“It took about a day to install the system, and then we spent the next day doing a calibration flight to make sure the LIDAR, GPS and camera were all working,” Sean Morgan said.
EagleScan based the aircraft at Klawock Airport on Prince of Wales Island where the differential GPS unit was set up on the control point. The project areas included 3,110 acres of forest units and 40 linear miles of road on Kosciusko and 2,143 acres of forest and 22 miles of road on Tuxekan. These forest units and roads were scattered throughout a 79.1 square-mile contiguous area on Kosciusko Island and a 27.7 square-mile area on Tuxekan.
All were flown in the last-return collection mode, except for a two-square mile site on Kosciusko, which was mapped twice, once in first-return and again in last-return mode.
Even though the Forest Service contract called for only mapping the scattered forest units and roads, EagleScan collected data over a much larger area. With very satisfactory results obtained in the target sites, the Forest Service and URS decided to process the LIDAR DEM and image data over the larger area of both islands. The total area of delivered data for both islands combined was about 240 square miles.
DATIS collected LIDAR and panchromatic image data on four flights over two consecutive days. Total flying time was 16 hours. After each flight, EagleScan technicians ran a quick pre-processing routine on the LIDAR data using a laptop computer to ascertain that acceptable data had been collected. Then the datasets were transmitted to the Colorado office for processing.
Assessing the Results“The sinkholes were easy to spot on the contour maps,” Langendoen said. “They look like big bulls eyes.”
In addition to delivering the map products to the Forest Service, URS distributed them to its field crews for use in assessing the timber stands and locating cave features. The accuracy of the maps was so good that the engineers began designing new roads on screen before they left the URS office for their field work.
“The maps didn’t eliminate field work, but they severely reduced the amount of time spent in the field,” Langendoen said. “Field work became a quick verification of what was seen on the maps so there aren’t any surprises when it comes time to build roads and harvest timber.”
He added that having the orthorectified images instead of just line maps contributed to the richness of information that could be interpreted while still in the office. For example, the existence of some features, such as streams, that could only be extrapolated on the contour maps was confirmed by examination of the imagery.
For Baichtal, who for years had relied on coarser-scale maps with 100-foot contours, the DATIS maps were like a new set of reading glasses.
“We just took the LIDAR topography maps, marked the points we wanted to investigate, and we’d walk right to them,” said Baichtal. “The accuracy was amazing.”
He assessed the LIDAR’s ability to identify surface features by comparing the new contour maps with aerial photographs from the 1950s and ’60s. Taken just after logging was completed, these photos revealed karst surface features. Since then, secondary growth had recovered the sites, but the LIDAR was able to penetrate and reveal the same details as the decades-old air photos.
In the small, two-square-mile area where two DATIS passes occurred, the resolution and accuracy of the resulting DEMs were slightly better than those where a single pass was made. This higher quality was due to the greater density of points collected.
“The benefit of dual-pass collection becomes significant with LIDAR when contour maps of 2' to 3' are produced,” Morgan said.
As expected, the Forest Service has purchased DEMs, contour maps and orthoimagery for all of Tuxekan and the southern half of Kosciusko from URS and EagleScan. According to Baichtal, this has offered some unexpected benefits.
“Instead of just looking at the areas in and adjacent to the proposed harvest sites, we are able to study the entire island as a system,” he said. “We can see the overall density of karst features and cave entrances and where springs are coming from. If we had that information originally, many of our earlier surveys would have been done differently.”
It’s too early to say how many karst features and trees will be set aside from harvesting on the two islands as a consequence of the LIDAR mapping, but URS believes the technology will change the way the Forest Service does business involving timber concessions.
“They can do a lot more planning in the early stages of a timber sale,” URS’ Every said. “This tool gives them the confidence to know exactly what to do with each stand of trees in their timber management plan.”