Testing the Waters
To allow for safe and efficient marine navigation, NOAA charts the near-shore and offshore waters (see glossary on page 30) of the United States. While these charts illustrate channels, buoys and other items that are critical to navigation, the most fundamental information on the map is the nature and shape of the seabed, the water surface, and the interaction between the tides and the land-sea interface. Currently, NOAA has an established listing of critical backlog areas around the United States, primarily in Alaskan waters, that need to be surveyed to current hydrographic charting standards. The data reaped from these surveys will then be used to update existing nautical charts.
Hydrographic surveying, which determines the configuration of the bottoms of water bodies, involves a good understanding of the ocean's dynamics, its interaction with the land and the seabed, and the influence of meteorological factors. Specialized instrumentation has been designed to accurately and optimally map waters, but few companies have the capability to undertake such contract jobs. In 2004, one company took on the challenge of surveying significant waters in southeastern Alaska with an approach never before attempted.
A Complicated ProcessThe ocean charting process is complicated, as different techniques are used for offshore and near-shore surveys, the sea-land interface and shoreline verifications. Fugro Pelagos Inc. (FPI; San Diego), a pioneer in airborne LiDAR bathymetry, met NOAA's charting needs by providing an integrated solution of multibeam sonar technology and LiDAR data services-the first such mix offered in the world. FPI is the only company in the world that owns both hydrographic LiDAR and multibeam systems. The company has been successfully performing contracted hydrographic surveys for NOAA since 1998.
"FPI continually strives to improve data collection and processing, and willingly shares breakthrough developments with the hydrographic community to better the field as a whole," says Edward J. Saade, president of Fugro Pelagos. "We have developed innovative and robust data acquisition and processing systems and methodologies that ensure the highest quality data and deliverables."
Early hydrographers used lead-weighted lines to aid their measurement of ocean depth. They threw the lines out of the boat and the weights hit the ocean bottom. The length of the line determined the water's depth, and sextant readings referenced to landmarks on shore determined the position of each "sounding," as the process is called.
Today's solutions, as FPI has proven, are much more advanced. Now, traditional hydrographic surveying for sea charts is done primarily with multibeam sonar (to measure water depths), a heave sensor (to compensate for ocean swells) and GPS technology (to determine horizontal position) while monitoring changes in tidal levels. FPI merged this traditional approach with LiDAR bathymetry technology in which a high energy laser delivers pulses to the ocean, retrieving returned data from the surface and the sea floor.
In August 2004, FPI tested its solution on an already mapped site in Shilshole Bay, Wash., that resulted in data well within NOAA hydrographic survey standards. Following this successful demonstration, NOAA officials approved the use of the integrated solution on the 20.7 square nautical miles off Sitka, Alaska, an historic and scenic community in the southeastern part of the state where definition of the seafloor's and shoreline's previously determined irregularities was incomplete.
The EquipmentThe survey area in Sitka was extremely complicated, containing a highly irregular shoreline with many rocks, foul areas and kelp, according to Robert Richards, PE, principal in charge of the project. "Hydrographic surveyors would not have been able to perform all the work with survey vessels alone. With limited ability to use conventional methods, the resulting shoreline would have been more ambiguous, and the delineation of rocks and kelp would not have been as thorough and accurate." Enter FPI's multibeam/LiDAR solution.
Prior to the survey conducted in Sitka, LiDAR and multibeam surveys were collected as stand-alone surveys and took at least two years to complete. The LiDAR data was collected the first year. The multibeam data, collected in the second year, included LiDAR data verifications. The survey conducted in Sitka was a trial to see if it was feasible to collect LiDAR and multibeam data in one project. The combined data collection proved to decrease the amount of time that NOAA needed to update the area's three nautical charts.
Due to the complexity of the area, this unique survey was completed in four phases using various survey platforms and equipment. The land-sea interface areas were flown with a fixed wing plane supporting a SHOALS (Optech, Toronto, Ontario, Canada) 1000T Airborne Laser Bathymeter. The SHOALS 1000T is capable of sensing the bottom to depths equal to 2.5 to 3 times the Secchi depth up to 50 meters. The Secchi depth is a measure of water clarity measured using a Secchi disk, a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Clarity is affected by algae, soil particles and other materials suspended in the water.
The system collected data from 15 meters water depth (WD) up to and including the shoreline. The near-shore data up to 100 meters water depth was collected from an R/V Quicksilver, a 35-foot survey vessel (Quicksilver Inc., Anchorage), which surveyed the area with a hull-mounted Reson (Goleta, Calif.) SeaBat 8101 multibeam echo-sounder (MBES). The deeper offshore data was collected from an R/V Kvichak Surveyor I (Kvichak Marine Industries, Seattle), a 67-foot catamaran equipped with a Reson SeaBat 8111 MBES. All LiDAR investigations and shoreline verifications were completed with a 15-foot skiff equipped with a CSI (CSI Wireless, Calgary, Alberta, Canada) GBX-Pro DGPS receiver, Fugro's WinFrog v3.4.0 data acquisition system and a digital camera. NOAA charts were displayed as a layer in WinFrog for reference. All soundings from the skiff on submerged features were conducted with a lead line.
The SoftwareFPI developed several software devices to enhance the quality of hydrographic data collection and processing. To aid in the processing of the shoreline verification results, FPI created a Shoreline Correlator add-on in ArcMap (ESRI, Redlands, Calif.) v8.2. It is the only integrated system that creates an individual Detached Position (DP) form, which includes the data log, tide, photos, NOAA chart (largest scale available), T-sheet (NOAA coastal survey map) data, smooth sheet soundings, multibeam coverage files, and now, photo-mosaics from the LiDAR system. Pelagos Precise Timing was developed as a new timing scheme, and allows data to be time-stamped when it is created, not when it's logged. The system uses a single clock/epoch to time-stamp all data. This new scheme was a precursor to True Heave technology, which FPI developed in 2003 with the assistance of Applanix (Richmond Hill, Ontario, Canada), CARIS (Fredericton, New Brunswick, Canada) and Triton Elics International (Triton Imaging, Watsonville, Calif.). True Heave offers numerous benefits including the reduction of timing errors, the diagnosis of mechanically induced artifacts, the broadening of the operational weather window, the reduction of the heave component in error budget, shorter turn times, line changes and overall easier shoreline surveys. Building upon the Pelagos timing development, FPI collected dual-frequency GPS data during 2004. This new technology was implemented in Applanix's POSPac post-processing software, CARIS HIPS bathymetric data cleaning and validation tool, and Triton Elics International's ISIS Sonar V6.5 and Delph Map software. The dual-frequency GPS data was post-processed for both the LiDAR and the multibeam system and is being evaluated as a tool for enhancing or replacing the need for installing multiple tide gauges in remote locations.
As this was the first time such a survey project had been attempted, some possible obstacles required streamlining. On all
prior hydrographic surveys utilizing LiDAR, explains Jana DaSilva Lage, the project's logistics and project manager, the LiDAR data was collected and fully processed to the smooth sheet stage during the first field season. At the end of the first field season, NOAA was unable to update the nautical chart with the LiDAR data, as there remained questionable soundings needing further investigation. The multibeam crew began the second field season with a survey area containing a zone of previously collected LiDAR data. They were required to compare the newly collected multibeam data with the LiDAR smooth sheet data and investigate a table of items identified by the previous contractor and NOAA. With only the "results" of the prior field season, it was difficult for the hydrographers to gauge the amount of effort they would need to apply to each item or what exactly they were to clarify. FPI's two crews, one reviewing LiDAR data and another reviewing multibeam data, were able to foster effective communication and coordinate data transfer. During the Sitka survey, two hydrographic teams collected data. "Both field crews shared the same office and were able to look at the aerial photographs along with the data in its most raw form," Lage says. "Working together this way, the multibeam and shoreline hydrographers were able to fully understand what the LiDAR hydrographers wanted them to clarify. This resulted in higher confidence that the entire area was covered and all questionable soundings were verified."
A Walk Through the ProcessDuring the LiDAR survey, the bathymetric laser was operated to achieve 4 x 4 meter spot spacing (equivalent to sounding density). Flying at an altitude of 400 meters and at approximately 160 knots, the crew achieved a 20 percent overlap of survey lines. This phase of the survey was flown twice, once at mean low water and once at mean high water in opposing directions to achieve 200 percent coverage. Topographic data was also collected applying the same parameters. A digital camera acquired one 24-bit color photo per second. The digital photos were used to assist in editing the data and creating an orthophoto mosaic, which allowed the hydrographers to better map out islets, rocks and kelp areas.
The SHOALS Ground Control System was utilized for flight planning, data processing and editing, quality control and data export. Marine and hydrographic solutions provider CARIS was contracted to write a converter program to bring SHOALS data and waveforms into the HIPS program for quality control; this also allowed the LiDAR data to be effectively merged with the multibeam data for production of smooth sheets (final hydrographic plots).
For the nearshore survey, a 32-foot R/V Quicksilver equipped with a hull-mounted Reson SeaBat 8101 multibeam system operated at a frequency of 240 kHz with 101 horizontal beams centered 1.5Â° apart (150Â° across-track beam width) and 1.5Â° along-track beam width. The system transmits and receives a sonar signal to measure the relative water depth over the 150Â° swath. Vessel attitude and position were measured using an Applanix Position and Orientation System for Marine Vessel (POS/MV). "Line orientation was generally parallel to the coastline and bathymetric contours in the area," explains Dean Moyles, lead hydrographer for the project. "The multibeam signal is projected in a fan shape. As the water depth increases, the seafloor coverage increases. Survey lines that were run perpendicular to the bathymetric contours would be inefficient. A survey in the deeper water would leave gaps between the lines as the vessel moved into shallower water. Likewise, a survey planned for full coverage in the shallow water would prove excessive as the vessel moved into deeper water. If the data were of poor quality, slow sound velocity or rough sea state, the line spacing would have been decreased to provide data meeting or exceeding specifications."
For the offshore survey, an R/V Kvichak Surveyor I, a 67-foot long catamaran specifically modified for hydrographic survey operations, was equipped with a pole-mounted Reason 8111 operated at a frequency of 100 kHz with 101 horizontal beams centered 1.5Â° apart (150Â° across-track beam width) and 1.5Â° along-track beam width. Vessel attitude and position were measured using a POS/MV also. Line orientation was generally parallel to the bathymetric contours in the area. Line spacing depended on the water depth and data quality, but never exceeded three times the water depth.
While the SHOALS system was performing the conformance test in Washington in August, the M/V Quicksilver was collecting survey data in the field with one of the multibeam systems. A subset of this data was used for QA/QC of the LiDAR data. Survey operations were based out of an office set up in Sitka, where data processing stations loaded with CARIS HIPS programs were set up. The LiDAR and multibeam crews collected data simultaneously, performing only daytime operations. The total survey time was three days for LiDAR operations and 30 days for the conventional multibeam survey operations.
For Safer WatersAccording to Lage, the only alternate way to complete the hydrographic survey in Sitka was by using conventional tide coordinated aerial photography and separate surveys with a LiDAR system and a multibeam system. "Such a survey would take several seasons to complete and would produce a less accurate product. The combined LiDAR/multibeam survey increases the accuracy of the final product, which is completed in far less time than conventional methods."
Overall, FPI learned that performing a joint multibeam LiDAR hydrographic survey is an efficient way to decrease the NOAA survey backlog by saving a season between LiDAR and multibeam data collection, thereby getting the data on to the nautical charts. The company's use of orthophotography allowed them to take their shoreline mapping capabilities to the next level while providing safer navigable waters.
Hydrographic GlossaryBathymetry: the underwater equivalent to topography.
Echo sounding: the use of sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water.
Foul area: an area that has a high concentration of rocks, kelp, wrecks or other debris that impedes navigation.
Shoreline: the line where water and land meet.
Hydrography: (1) the science comprising the description, study and mapping of the waters of the Earth's surface, including their forms and physical features. (2) the subject matter of this science, the hydrographic features of the globe or part of it; the distribution of water on the Earth's surface.
LiDAR (light detection and ranging or laser imaging detection and ranging): a technology that determines distance to an object or surface using laser pulses.
Mean high water: a tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch (19 years). For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.
Mean low water: a tidal datum. The average of all the low water heights observed over the National Tidal Datum Epoch (19 years). For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.
Nearshore: the zone that extends from the shoreline to the position marking the start of the offshore zone, typically at water depths on the order of 20 meters.
Offshore: the comparatively flat zone of variable width, extending from the nearshore zone to the edge of the continental shelf. It is continually submerged.
Shoal: a sandbank or sandbar that makes the water shallow; specifically, an elevation that is not rocky and on which there is a depth of water of six fathoms (11 meters) or less.
Skiff: typically a small flat-bottomed open boat with a pointed bow and square stern.
SONAR (sound navigation and ranging): a device that uses reflected sound to determine positions of objects, similar to radar, which uses reflected radio waves. See echo sounding above.
Smooth sheet: the final, neatly drafted, accurate plot of a hydrographic survey, which depicts shoal-biased soundings selected from the hydrographic records.
T-sheet data: data from a NOAA Coastal Survey Map.
Waveform: the shape of the laser signal return illustrated graphically by plotting the values of the period quantity against time.
Sidebar: Surveying WreckagesWith hydrographic surveys, NOAA played a crucial role in finding the wreckage of TWA Flight 800 into the Atlantic Ocean off Long Island, N.Y., in July 1996;
John F. Kennedy Jr.'s plane off the coast of Martha's Vineyard in July 1999; and EgyptAir Flight 990 off the coast of Nantucket Island in November 1999.