Aerial Perspective: Pribilof Islands, Alaska; LiDAR Survey Project

Aerial Data Acquisition in Harsh and Remote Conditions

Alaska's remote Pribilof Islands endure some of the roughest storms in the world. Although the temperature range of the marine climate is narrow, usually between 19 and 51 degrees Fahrenheit, some storms push the temperature below zero with winds in excess of 60 knots. The islands are often beset by storms in winter and persistent fog in summer, making a difficult surveying environment. The town of St. George on the northeast shore of the 35-square-mile St. George Island is one of five Pribilof islands that are north of the Aleutian chain and 750 miles west of Anchorage. The St. George harbor that supports the Bering Sea fishing fleet and is critical to the local economy was extensively damaged by severe winter storms in 2004.

Alaskan engineering firm, Peratrovich Nottingham & Drage (PND, Inc.), was tasked to design and supervise repairs. PND contracted with the Anchorage office of AERO-METRIC, Inc. to map the harbor and provide detailed models of the breakwaters that are often washed over by waves even between storms. Long experienced in working in challenging environments and always seeking innovative solutions, PND specified the use of airborne LiDAR technology. This approach would mean that ground-based surveyors would not have to risk working on the dangerous breakwaters to capture the dense dataset required to design the reconstruction.

A Brief History of Saint George

In 1786, Gavrill Pribilof, of the Russian Lebedov Lastochkin Company, discovered St. George while looking for the northern fur seal breeding grounds. He named it "Sveti Georgiy." The Russian American Company relocated Aleut hunters from Siberia, Unalaska, and Atka to St. George and sister island, St. Paul, to harvest fur seal. In 1867 the U.S. purchased the territory of Alaska from Russia, and between 1870 and 1910 the U.S. Government leased the Pribilofs to private companies, which provided housing, food, and medical care to the Aleuts in exchange for their work in the fur seal plant. In 1910, the U.S. Bureau of Fisheries took control of the islands, but shortly thereafter the cumulative effects of over-harvesting nearly destroyed the industry and depressed the tenuous local economy.

Sitting on the edge of the continental shelf, the Pribilofs benefit from southwest currents that bring nutrient-rich waters to the surface, fueling the food chain from the deeper Aleutian Basin. Currently, residents are striving to develop commercial fisheries and to attract tourists who come to see the huge populations of marine mammals and the more than 240 species of birds that nest on the island's cliffs.

Project Details

AERO-METRIC assigned to the project a two-man crew in a LiDAR-equipped twin-engine Cessna 320. The crew flew two data acquisition missions, in each case basing out of Dillingham, approximately 400 miles to the northeast on the mainland (accommodation is in short supply at St. George, and Dillingham is still close enough to allow for a quick response to short weather windows). The goal of the first flight mission, carried out in May of 2005, was to collect data for a site survey and damage assessment. This data was merged with bathymetric data supplied by the client, which was helpful in showing the locations of probable change in ocean floor profiles outside of the breakwaters, as well as on the harbor bottom within the breakwaters. Next, the engineering team designed repairs, and the local authorities hired contractors to reconstruct the breakwaters.

The second flight mission, flown in November 2006 after breakwater repairs were completed, had two goals: to provide an as-built survey and to verify the quantities of new armor rock actually placed by the contractor.

Both missions were flown at 500 meters Above Mean Terrain (AMT), with intersecting flight lines for redundancy and quality checking, in order to accurately model the riprap on the breakwaters. The client engaged Terrasond, Ltd. of Palmer, Alaska to provide ground control for the first mission and dispatched an AERO-METRIC field crew for the second mission. The raw LIDAR data points for the DEM product were captured using an OPTECH 30/70 ALTM 70 kHz LIDAR system. The system includes differential GPS (DGPS) and an inertial measurement system (IMU) to provide the required geospatial reference. Nominal datapoint spacing was 0.6 meters. The deliverable was a bare-earth point cloud DEM at a density and vertical accuracy suitable for 2' contour interval topographic mapping.

LiDAR Data Processing

The ALTM-NAV on-board software that controls the operation of the LiDAR system provides the airborne operator with a near-realtime view of the data acquired to ensure complete coverage of the area of interest. On the St. George project, the post processing of the GPS, IMU, and LiDAR vector data was performed with Optech's REALM software. This yielded the "Smoothed Best Estimated Trajectory" and ultimately the surface-defining point cloud.

TerraSolid's TerraScan software was used to create a "bare earth" data set by removing the returns from the few buildings and scant vegetation.

The software also removed the majority of the returns from the many waves created by high winds. With this step complete, a rigorous manual edit by technicians was conducted with considerable time spent in manually classifying the breakwaters for detailed definition. These data were then post-processed using TerraMatch software that mathematically computed attitude corrections to ensure data consistency.

Perspective on the Value of Using LiDAR

Troy Gere, GIS Manager at PND, summed up his view of PND's choice of LiDAR technology for this challenging project: "Given the remote location, extreme conditions and tight timeline we chose to use LiDAR to meet the project requirements. The level of detail that LiDAR technology provided us far exceeded what we could have obtained using any conventional ground survey methods in the hazardous working environment on the breakwaters for St George harbor. Also, the data acquisition time was reduced from one or two weeks to just two days using LiDAR. We have used the technology in the past and will continue to depend on LiDAR in the future for our projects in the remote locations and harsh environments of Alaska."

About the Author

Mark Syren is a senior project manager with the Anchorage, Alaska office of Aero-Metric, Inc. He can be reached at 907-272-4495 or msyren@aerometric-ak.com.

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