Feature ArticleAirborne Lidar Bathymetric and Hyperspectral Surveys in the Caribbean
By Mark Sinclair
Fugro LADS Corp. Pty Ltd.
Final coverage of the Puerto Rico survey.
Demonstrating a wide range of applications, a number of major airborne lidar bathymetry (ALB) projects have been conducted using the Fugro laser airborne depth sounder (LADS) system. Covering an area of greater than 3,000 square kilometers, the Fugro system was used in Caribbean waters offshore Puerto Rico in 2006, with subsequent surveys in Martinique, Guadeloupe and the U.S. Virgin Islands in 2010 and 2011.
These projects have included the measurement of depth for nautical charting and other environmental or coastal zone management requirements, as well as the mapping of seabed habitat using reflectivity data calculated from the ALB waveforms.
Digital georeferenced mosaicked imagery and hyperspectral data in the visible to near-infrared spectral ranges have also been collected. The airborne data have been supplemented with ground truth data, including seabed sampling, spectroscopy measurements made within the water column and on the seabed, and the collection of long-term tide data.
The projects were conducted for a number of customers, including NOAA's Office of Coast Survey for a survey on the west coasts of Puerto Rico, St. Thomas and St. John, and the Service Hydrographique et Océanographique de la Marine (SHOM) for its Litto3D project around the islands of Martinique and Guadeloupe. The data from these projects have been used by a variety of agencies and organizations to improve the nautical charting coverage and support a number of coastal zone management applications such as environmental monitoring and habitat mapping.
Furthermore, a research project at St. Croix was conducted for a number of agencies, including NOAA's Center for Coastal Monitoring and Assessment, the U.S. National Parks Service and the University of New Hampshire.
Operating Sites, Schedule and Environmental Conditions
The survey of the western end of Puerto Rico in 2006 was conducted as a single project from a base in San Juan, Puerto Rico. At this northerly part of the Caribbean chain, the environmental conditions were highly suitable for an ALB survey. This is also true of the St. Thomas, St. John and St. Croix survey areas in the U.S. Virgin Islands, which are relatively close to Puerto Rico. Vessel reconnaissance at St. Thomas and St. John recorded average depths measured by Secchi disk of 42 feet and maximum Secchi depths of 60 feet. These measurements were taken during the wet season in August and indicate that these areas remained clear at most times.
In 2010 and 2011, operations were conducted in Martinique, Guadeloupe and the U.S. Virgin Islands. The southeast end of the Caribbean chain was found to experience more adverse environmental conditions with significantly more clouds at lower levels, higher rainfall and higher levels of turbidity. A more flexible approach had to be adopted and survey operations were conducted concurrently in these three areas from bases at San Juan, Fort de France, Martinique, and Pointe-a-Pitre, Guadeloupe. This flexible approach of moving the aircraft and system between alternative survey areas optimized conditions for survey.
The window of suitable environmental conditions for ALB in the Caribbean reduces as one heads southeast. The window is virtually all year in the U.S. Virgin Islands, from the start of March to possibly the end of June in Guadeloupe and from mid-March to mid-June in Martinique. The ability to conduct numerous projects in a similar geographic area concurrently enabled the ALB system to be used to survey each area when environmental conditions were most suitable.
Interim coverage of the Martinique survey.
Horizontal and Vertical Control
The real-time horizontal control for all surveys was the World Geodetic System 1984, and the NOAA surveys were post-processed onto North American Datum 83. Real-time positions were determined using an Ashtech SAS (Carquefou, France) GG24 GPS receiver aided by a wide area differential GPS service. A local GPS base station was also established in each survey area, and government-operated continuously logging GPS services were also utilized, including NOAA's National Geodetic Survey Continuously Operating Reference Station. These local logging stations enabled post-processed kinematic GPS positions to be determined relative to the local GPS base station and then applied to all soundings. This provided increased sounding position accuracy and horizontal redundancy.
Vertical control was achieved using observed tides relative to mean lower low water for the NOAA surveys and for the SHOM surveys, relative to a vertical coordinate system defined by IGN France (IGN 1987) for Martinique and mean sea level along with Geodetic Reference System 1980 for Guadeloupe. A number of existing permanent tide gauges were used, which were supplemented by additional temporary gauges as required. Checks were conducted on the tide gauges and data including leveling to benchmarks, 25-hour pole-gauge comparisons and smoothing of the data. The reduction of soundings for tides throughout the survey area was then conducted through tide models, including linear and planar interpolations and establishing tide zones (polygons) where factors were applied to the height of tide and offsets to the time within each individual zone.
For all surveys, vertical control of soundings was checked by surveying a number of flat areas of seabed on each flight and conducting crosslines throughout the survey areas. Mean depth differences and standard deviations between the data sets were determined. These checks demonstrated the data were consistent with International Hydrographic Organization Order 1 depth accuracy.
Puerto Rico Survey
The primary requirement of the Puerto Rico survey was to provide NOAA with modern, accurate hydrographic survey data with which to update nautical charts. A secondary requirement was to provide relative seabed reflectivity for habitat mapping.
The survey area was sounded at four-by-four-meter laser spot spacing with main lines of sounding spaced at 80 meters, which provided 200 percent coverage. Some 21 survey sorties of nominal seven-hour duration were flown. The survey operations were conducted in wind strengths of up to 20 knots, the sea state ranged from 1 to 3 and long-period swell was not experienced. Cloud levels would build up over land and move offshore during the early afternoon, which increased toward the end of the survey. These environmental effects did not adversely affect the data quality. Maximum depths to 55 meters were measured before the reef dropped off quickly, and consistent depths of 35 to 40 meters were achieved throughout the survey area. This area was previously surveyed by National Ocean Service, partly between 1900 and 1939 by lead line, and partly between 1940 and 1989 by single-beam echosounder.
During the most recent survey, 47 new dangers to navigation (DTON) were found and differences between the survey and the existing charts identified. A DTON is an inadequately charted feature that a field hydrographer identifies as potentially dangerous to navigation, taking into account the general vessel traffic and largest scale chart produced for the area.
Some examples of DTON as outlined by NOAA's Office of Coast Survey include: submerged features with depths less than 11 fathoms (66 feet) in navigable waters; items found to be significantly shoaler than charted; incorrect or uncharted clearances on bridges or overhead cables; and floating or fixed aids off position or incorrectly labeled.
In addition to the specific DTON, a large number of other significant differences between the survey and charts were also identified and reported for each of the survey areas. Although not as notable as DTON, these items of significant difference also demonstrate the importance of conducting modern surveys to improve the knowledge of the area and enhance navigational safety. The total number of DTON and significant differences totaled 1,099 items in the 675 square kilometers that were surveyed in Puerto Rico.
In addition to DTON and chart differences, a number of general recommendations regarding differences between the survey and chart were made for certain parts of the survey area. These included differences between the surveyed and charted coastlines by up to 150 meters, cultural details such as buildings, wharfs, recreational jetties and seawalls, which were not shown on the chart or where the charted position differed by up to 80 meters. Some of these differences between the survey and the chart were of greater or lesser importance, but they serve to demonstrate the huge amount of detail which has been surveyed and was not previously correctly shown on the chart. To continue this article please click here.
Cmdr. Mark Sinclair (retired) is the managing director of Fugro LADS Corp. Pty Ltd. He has more than 30 years' experience in the marine environment, 17 involving airborne lidar bathymetry projects. He holds a degree in physical oceanography from the University of New South Wales, a graduate degree in land-data management from the Royal Melbourne Institute of Technology and is an IHO level 1 certified hydrographic surveyor.