More information about the individual USGS surveys conducted as part of the Buzzards Bay project can be found on WHCS Field Activity Web pages:
2009-002-FA: <http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2009-002-FA> 2010-004-FA: <http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2010-004-FA> 2011-004-FA: <http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2011-004-FA>
Information about the NOAA survey can be found at:
H11319: <http://surveys.ngdc.noaa.gov/mgg/NOS/coast/H10001-H12000/H11319/DR/>
Soundings from the USGS surveys were processed and edited using Computer Aided Resource Information System (CARIS) Hydrographic Information Processing System (HIPS; ver. 6.1 and 7.1). Although the soundings were manually edited, small data spikes may exist at outer edges of some swaths. In addition, small gaps between adjacent survey lines may exist, especially in depth less than 14 meters. Tielines and transit lines were generally excluded from this grid if quality main survey line data were available. The tieline and transit line data were used to verify tide corrections during post-processing.
NOAA's CARIS directory for survey H11319 was provided to the USGS. No additional swath processing or navigation editing was performed to the NOAA data in CARIS.
Field activities 2009-002-FA and 2010-004-FA: Navigation was acquired with a Coda Octopus F180 Differential Global Positioning System + Wide Area Augmentation System (DGPS+WAAS), which is accurate to + or - 1 to 2 meters, horizontally. All DGPS data are referenced to WGS84. The SWATHPlus transducers were mounted on a rigid pole, approximately 2.46 m below the water line, along the starboard side of the M/V Megan T. Miller. The DGPS antenna was located on the same pole approximately 4.3m above the sea surface. These offsets were corrected within the SWATHPlus acquisition software.
Field activity 2011-004-FA: Navigation was acquired with a Coda Octopus F180 Differential Global Positioning System + Wide Area Augmentation System (DGPS+WAAS), which is accurate to + or - 1 to 2 meters, horizontally. All DGPS data are referenced to WGS84. The SWATHPlus transducers were mounted on a rigid pole, approximately 2.17 m below the water line, along the port side of the M/V Scarlett Isabella. The DGPS antenna was located on the same pole approximately 4.6m above the sea surface. These offsets were corrected within the SWATHPlus acquisition software.
The DGPS horizontal positional accuracy is estimated to be within 3-5 m; WAAS enabled DGPS accuracy is estimated to be within less than 3 m and the accuracy of RTK-GPS (which was not applied to the horizontal positions for these surveys) is estimated to be less than 1 m. However, the U.S. Coast Guard maintains a maritime DGPS service providing 10 meter accuracy for the coastal areas of the continental US; therefore, a very conservative, worse-case estimate of the horizontal positional accuracy of this survey data is approximately 10 meters (although these systems have been shown to be reliable to less than 2m). (sources: <http://www.noaa.gov>; <http://www.nos.noaa.gov>; <http://www.ngs.noaa.gov>; <http://www.nstb.tc.faa.gov/>; <http://www.navcen.uscg.gov>).
USGS field activities 2009-002-FA, 2010-004-FA and 2011-004-FA: Vertical accuracy of the raw data based on system specifications may approximate 1% of water depth, 0.1 to 0.4 meters within the survey area. However, overall vertical accuracies on the order of 50cm or better are assumed based on the following considerations: The Coda Octopus F180 Attitude and Positioning system, used to correct for vessel roll, pitch, heave, and yaw, has a theoretical vertical accuracy of a few mm. Tidal offsets were corrected to MLLW using Real-Time Kinematic GPS (RTK-GPS) heights in post-processing (see process steps). Corrections were sent via cellular modem and VHF radio to the ship from the base station on land. The primary reference station is located at the USGS Marine Operations Facility (MOF) in Falmouth, MA. USGS field tests (unpublished) using submerged targets suggest that the vertical accuracy of the RTK-GPS tidal correction is less than 30cm. Refraction artifacts were minimized by acquiring a range of sound velocity profiles with a hand-casted Applied MicroSystems SV Plus sound velocimeter during survey 2009-002-FA and an ODIM MVP30 moving vessel profiler during surveys 2010-004-FA and 2011-004-FA. Sound velocity data are entered into the SWATHPlus acquisition software to model the sound velocity structure of the water column. Gridding algorithms and cell sizes for these data could introduce errors as great as 3m along the edges of the data, but gridding-induced errors are likely much smaller (less than 20cm) for most of the survey area. Changes in ship draft due to water and fuel usage were not considered.
USGS Configuration: Survey: Survey lines were run at an average speed of 5 knots and were spaced 50-100 m apart to obtain overlapping swaths of data and full seafloor coverage. Sonar: The SEA SWATHplus-M operates at a frequency of 234 kHz and a variable range (increased or decreased manually depending on water depth). The system was operated with a transmit power: 80 percent, a transmit length: 12 cycles (2009-002-FA and 2010-004-FA) or 43 cycles (2011-004-FA), 3072 samples per channel (2009-002-FA) or 4096 samples per channel (2010-004-FA and 2011-004-FA), and a variable range (increased or decreased depending on water depth). Speed of Sound: Sound-velocity profiles were collected approximately every 2 hours by hand casted Applied MicroSystems SV Plus sound velocimeter (2009-002-FA), or an ODIM MVP30 moving vessel profiler (2010-004-FA and 2011-004-FA). Tides: Tides were typically corrected during post-processing using one- or two-minute interval averages of the RTK tide data recorded in the "TID" message string of the HYPACK navigation file for each line (however; RTK signal dropouts during survey 2009-002-FA and 2010-004-FA made it necessary to post-process the base and rover navigation data using NovAtel GrafNav (version: 8.30.1123) to calculate the RTK tide offset values, see process steps for more details).
While much effort was devoted to cleaning the data and minimizing survey artifacts, some artifacts may still be seen in the final gridded data including an along track artifact at nadir (the ship track) and at the far edges of a survey line where adjacent swaths overlap. These artifacts are especially noticeable in areas of little local relief.
NOAA Configuration: Survey: All data were collected by the NOAA Ship RUDE. Trackline spacing for survey H11319 was based on a estimate of 2.5 times the water depth to achieve overlapping swaths for full seafloor coverage. For this area survey lines were typically between 25-50m apart. Ship speed was determined such that the survey achieved a sampling rate of 3.2 pings per 3 meters of along track distance. Sonar: The RESON SeaBat 8125 is a 0.5 degree (across-track) by 1 degree (along-track) beam angle system which operates at a frequency of 455 kHz and collects 240 individual soundings in a swath oriented perpendicular to the vessel track. The system is capable of ping rates ranging from 3.02 Hz to 40.05 Hz but for this survey, the maximum ping rate was set to 20 Hz. Speed of Sound: SeaBird SBE 19 SEACAT conductivity, temperature and depth (CTD) profiler was used to measure sound-velocity data. Tides: The tidal datum for survey H11319 is Mean Lower Low Water (MLLW). Tidal corrections were calculated using verified data from tidal station #8452660 at Newport, RI, and final corrected soundings (from a zoned tidal model) were referenced to MLLW. Other: All bathymetry processing, including tide-correction, were done by NOAA Hydrographers. NOAA's Computer Aided Resource Information System (CARIS) directory was provided to the USGS. Refer to the Fall Data Acquisition and Processing Report (DAPR) and the Horizontal and Vertical Control Report, for detailed equipment, vessel configuration and setting information.
Only the data necessary to fill gaps in the 2009-2011 USGS bathymetry survey data were used. Although the CARIS database contains the "raw" bathymetry soundings, it also contains any of the soundings and navigation edits, refraction and tide corrections, and motion reference unit offsets that were done by NOAA hydrographers. All NOAA-provided edits, corrections, and offsets were used in the final bathymetry grid for Buzzards Bay. No additional editing was done to the NOAA-provided data.
CARIS processing for NOAA hydrographic survey H10993: The CARIS HIPS directory was received from the National Oceanic and Atmospheric Administration - National Geophysical Data Center (NOAA-NGDC). The multibeam bathymetric data from NOAA was delivered fully processed and tide-corrected by NOAA Hydrographers. The CARIS HIPS project was reviewed and checked for any inconsistencies. Survey lines that were not needed to create a complete gridded surface for the Buzzards Bay survey area were removed from the project.
However; RTK signal dropouts during parts of surveys 2009-002-FA and 2010-004-FA made it necessary to post-process the base station and rover navigation data using NovAtel GrafNav (version: 8.30.1123). This process calculates the RTK tide offset values, using the height data recorded separately by the Master/Base station (which was set up at the USGS Marine Operations Facility) and the Remote/Rover station (the GPS antennea on the survey vessel). RTK tide data processed with GrafNav were then formatted for input into CARIS using a Perl script parse_GrafNav_files.pl. The tide (.tid) files for each survey were finalized by using data from either the GrafNav processed or the HYPACK extracted tide data. In Buzzards Bay: RTK tidal corrections for Survey 2009-002-FA were post-processed from base-station and rover data using GrafNav, except for discrete periods of JD152 (1200-2359), JD158 (0000-1516 and 2353-2359), JD159 (0000-2400), and JD160 (0000-0145) where HYPACK extracted tides were used. Only HYPACK extracted tide data were used for Survey 2010-004-FA. Only HYPACK extracted tide data were used for Survey 2011-004-FA. RTK tide corrections were loaded and applied to the soundings line by line in CARIS. Each line was re-merged and the preliminary BASE surfaces were recomputed to incorporate the new tide information. Additional swath editing was done in CARIS using the swath and area based editors to remove any additional inconsistencies and minimize survey artifacts.