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Bathymetric Lidar Survey Of Penghu Islands and Dongsha Atoll
Using an Ellipsoidal Height System for Bathymetric Mapping In Shallow Waters and Difficult-to-Navigate Environments

Peter Tian-Yuan Shih
Natural Hazard Mitigation Research Center
National Chiao Tung University
Hsinchu, Taiwan
Dushan Arumugam
Hydrographic Technical Manager
Woolpert Inc.
Denver, Colorado

Shiahn-Wern Shyue
Associate Professor
Department of Marine Environment and Engineering
National Sun Yat-Sen University
Kaohsiung, Taiwan

Sonar instruments, either single-beam or multibeam, are the major technology for bathymetric mapping, offering potentially higher spatial resolution and better depth accuracy. However, for areas with shoals or navigational risks, airborne bathymetric lidar has the advantages of higher efficiency and much better navigation feasibility.

Taiwan's Ministry of the Interior supported a project utilizing this airborne bathymetric lidar to map the Dongsha Atoll and a portion of the Penghu Islands off the western coast of Taiwan in the Taiwan Strait. This project had two major objectives. The first was to evaluate airborne lidar bathymetric technology and develop a strategy to adopt this technology for future shallow-water mapping. The other was to establish seafloor terrain information for marine research in Dongsha and potential engineering applications in Penghu.

This article outlines the data acquisition, processing and validation of this survey. Based on the cross-check between the overlap of flight lines and cross-flight lines, the result meets the International Hydrographic Organization's Order 1B specification.

Survey Areas
The Penghu Islands are an archipelago of more than 90 islands covering about 127 square kilometers. Magong, the capital of Penghu, features unique geological and marine settings, including Taiwan's second marine national park, which is comprised of four islands in southern Penghu.

The Dongsha Atoll, located 480 kilometers southwest of Kaohsiung, forms a nearly perfect ring, with a diameter of about 25 kilometers. Taiwan designated it as the country's first marine national park in 2007. Dongsha Island, measuring 2,800 meters long and 865 meters wide, is the only land that is constantly above water in the Dongsha Atoll. Its highest elevation is about 7.8 meters.

The survey was conducted in 2010 in August and September using a Hawkeye II lidar system manufactured by Airborne Hydrography AB (Jönköping, Sweden) that was installed on a SA-226T aircraft. The Dongsha survey area was about 506 square kilometers and was completed with 183 flight lines and 22 flying hours. The Penghu survey covered about 415 square kilometers from 118 flight lines using 19 flying hours.

Bathymetric lidar coverage in Penghu is shown. Due to turbidity, the bathymetric lidar points from flight lines on the left-hand side of Penghu's main island are fewer than those flight lines on the right-hand side.

The team also surveyed an area of 20 square kilometers in the east portion of the Dongsha Atoll. The survey was conducted at a two-by-two-meter resolution from a height of 300 meters, using 35 flight lines over six flying hours. The swath width of this point density was 120 meters.

The Bathymetric Lidar System
In the planning stage, which began in December 2009, selecting an airborne platform was a major issue. Due to the complexity of importing a surveying aircraft from overseas, great efforts were put on evaluating domestic airborne platforms, both fixed wing and rotating wing. Several constraints from the airborne bathymetric lidar instrumentation, namely, the required floor area, the cooling capabilities, the flying speed and the aircraft's power capacity, ruled out all the commercially available options in Taiwan. While the government-owned Beechcraft King Air 200 and 350 aircrafts were good candidates, their busy schedules prevented further arrangement. As a result, a dedicated aircraft, the SA-226T, was imported into Taiwan for the bathymetric lidar mission.

The survey was conducted with the HawkEye II lidar system, which uses an airborne laser that detects the seafloor in shallow coastal waters up to 50 meters deep. It has four channels: an infrared channel, two green channels and one channel for Raman scattering. The HawkEye II system simultaneously collects four-kilohertz bathymetric lidar soundings, 64-kilohertz topographic points and digital images acquired at one hertz covering the lidar-scanned area.

The HawkEye system includes an IDE UI-2250SE digital camera with a 1,200-by-1,600-pixel resolution and focal length of 12 millimeters. Full waveform information was stored for the bathymetric laser. The laser fired from HawkEye II maintains an angle of approximately 20' from nadir, which allows it to avoid direct reflection from the sea surface.

Data Acquisition
The HawkEye II system arrived in Taiwan in August 2010. The team conducted a calibration flight at beginning and the end of the survey. The test flights and calibration verification were all carried out at Penghu. The point density for both the Penghu and Dongsha areas was about 3.5 by 3.5 meters, done from a flight height of 400 meters and survey speed of 150 knots. The swath width of this point density was 160 meters. The overlap between flight lines for the whole survey was 20 percent.

In Dongsha, the flight lines were oriented in a north-south direction to minimize the effects of sun glint. The survey of Dongsha included the complete atoll and extended out to 50 meters' water depth or laser extinction, whichever came first. During the Penghu data acquisition, a temporary base of operations for processing and validating the data was located in Kaohsiung, which is about 200 kilometers from Penghu. For the Dongsha Atoll surveys, Dongsha Airport was used as the base of operations, and a temporary base in Dongsha was used for processing and validating the data.

Data Processing and Editing
During data acquisition, on-site data processing was conducted in Kaohsiung and at the temporary Dongsha base. A data analyst solved and reviewed the lidar point clouds and then made decisions of whether it was necessary to adjust or refly the route. Data post-processing and validation were held in National Chiao Tung University's laboratory.

The first part of data calibration is the internal calibration of the HawkEye II, which is carried out in the manufacturer's laboratory. This calibration allows for errors within the unit and includes the laser alignment, internal scanner angle and latency between components. The second step in data calibration is conducted and verified in the field and is concerned with orientation errors, particularly between the inertial motion sensor and the center of the scanning mirror. The boresight calibration area was located in western Penghu, which was 0.5 square kilometers of flat ground and about 2.2 kilometers line over sea, with a flight attitude of 300 and 400 meters. To continue this article please click here.

Peter Tian-Yuan Shih is a professor at the Natural Hazard Mitigation Research Center at National Chiao Tung University in Taiwan. He received his Ph.D. degree from the University of New Brunswick's Department of Surveying Engineering in 1989.

Dushan Arumugam is the hydrographic technical manager for Woolpert Inc. He previously was the co-owner and technical manager of Geomatics Data Solutions in Surrey, Canada, before it was acquired by Woolpert. He has served as lead hydrographer on numerous multibeam and lidar projects across five continents since 1999.

Shiahn-Wern Shyue is an associate professor at the Department of Marine Environment and Engineering of National Sun Yat-sen University in Kaohsiung, Taiwan. He received his Ph.D. from Department of Civil Engineering at the University of Washington. His research interests involve single-beam and multibeam bathymetric survey and mapping, underwater features identification, coast and marine GIS.

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