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The surveys were conducted using the research vessel R/V Parke Snavely outfitted with an interferometric sidescan sonar for swath mapping and Real-Time Kinematic navigation equipment for accurate shallow water operations.
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The bathymetric surveys were conducted using a 234.5 kHz SEA (Systems Engineering & Assessment Ltd) SWATHplus-M phase-differencing sidescan sonar. The sonar was pole-mounted on the 34-foot USGS mapping vessel R/V Parke Snavely, and affixed to a hull brace. Real-time kinematic (RTK) GPS position data were passed through a CodaOctopus F180 intertial measurement unit (IMU) to the sonar hardware and data collection software. Sonar heads, GPS antennae, and the IMU were surveyed in place to a common reference frame with a Geodimeter 640 Total Station. The R/V Snavely was outfitted with three networked workstations and a navigation computer for use by the captain and survey crew for data collection and initial processing.
Geodetic control for the survey was established using a shore based Global Positioning System (GPS) base station broadcasting Real Time Kinematic (RTK) corrections to the survey vessel via UHF radio link. The base station was located at the base of Ediz Hook. The base station was programmed using the WGS84 (G1150) reference frame with an Epoch of 2010.1548. Average Opus Solution coordinates for the station are:
Reference Frame: WGS84 (G1150) Epoch: 2010.1548 Latitude: N 48° 08 06.01943 Longitude: W 123° 28 06.90674 Ellipsoid Height: -11.355m The average values for the derived OPUS solution for MILL are: Reference Frame: ITRF00 [same as WGS84 (G1150)] Epoch: 2010.1548 Latitude: N 48° 08 06.01936 Longitude: W 123° 28 06.90685 Ellipsoid Height: -11.351m The differences to be added to the RTK broadcast locations are: Latitude: -0.00007 Longitude: +0.00011 Ellipsoid Height: +0.004m Using UTM coordinates as a comparison: The base station was programmed using the following coordinates: Reference Frame: NAD83, UTM zone 10 Epoch: 2002.0000 Northing: 5331411.376m Easting: 465138.251m Ellipsoid Height: -11.042m Orthometric Height: 9.155m (based Geoid09)
The R/V Snavely was equipped with a CodaOctopus F180 attitude and positioning system for the duration of the survey. The F180 is running F190 firmware, and receives real-time kinematic (RTK) corrections directly. The RTK GPS data (2 cm error ellipse) are combined with the inertial motion measurements directly within the F190 hardware so that high-precision position and attitude corrections are fed in real-time to the sonar acquisition equipment. The WGS84 (G1150) Epoch 2010.1548 3-dimensional reference frame was used for all data acquisition.
Sound velocity measurements were collected continuously with an Applied Micro Systems Micro SV deployed on the transducer frame for real-time sound velocity adjustments at the transducer-water interface. The Micro SV is accurate to +/- 0.03 m/s. In addition, sound velocity profiles (SVP) were collected with an Applied Micro Systems, SvPlus 3472. This instrument provides time-of-flight sound-velocity measurements by using invar rods with a sound-velocity accuracy of ±0.06 m/s, pressure measured by a semiconductor bridge strain gauge to an accuracy of 0.15 percent (Full Scale) and temperature measured by thermistor to an accuracy of 0.05 degrees Celsius (Applied Microsystems Ltd., 2005).
GPS data and measurements of vessel motion are combined in the F180 hardware to produce a high-precision vessel attitude packet. This packet is transmitted to the Swath Processor acquisition software in real-time and combined with instantaneous sound velocity measurements at the transducer head before each ping. Up to 20 pings per second are transmitted with each ping consisting of 2048 samples per side (port and starboard). The returned samples are projected to the seafloor using a ray-tracing algorithm working with the previously measured sound velocity profiles in SEA Swath Processor (version 3.05.18.04). A series of statistical filters are applied to the raw samples that isolate the seafloor returns from other uninteresting targets in the water column. Finally, the processed data is stored line-by-line in both raw (.sxr) and processed (.sxp) trackline files. Processed (.sxp) files were further processed with sxpegn (build 151) by David Finlayson (USGS) to remove erroneous data from the files and make valid gain-normalized amplitude data for processing backscatter data.
The digital elevation model (DEM) produced in this work is solely derived from the bathymetric data collected by the USGS during field activity S-6-10-PS. CARIS HIPS and SIPS (version 7.0.2 Service Pack 2) was used to clean and bin the raw bathymetry. Processed .sxp files were imported to CARIS, and field sheets were created within CARIS to encompass the survey area.
Survey lines were filtered to remove adjacent line data from nadir gaps. A CARIS Swath Angle BASE (Bathymetric with Associated Statistical Error) surface was created at 1 m resolution and the subset editor was used to clean artifacts and other unwanted soundings. The binned data were exported as an ASCII table along with calculations of the bin standard deviation (of all soundings within the 1 m x 1 m cell spacing) and the sounding density.
To convert the data from ITRF2000 (also known as WGS84 G1150) ellipsoid heights to orthometric heights on NAD83 (CORS96)/NAVD88, the Caris gridded data was exported as an ASCII table. The data was next transformed from the the ITRF2000 ellipsoid to the NAD83 (CORS96) ellipsoid using a 14-point Helmert transformation using the parameterization described in Soler and Snay (2004) with the command line tool CS2CS in the Proj4 library (<http://trac.osgeo.org/proj/>). The parameters were calculated for an Epoch date of 2010.1548. The conversion from WGS84 (G1150)/ITRF2000 to NAD83 (CORS96) shifts the data about +0.90 m to the east, -0.80 m to the south, and +0.30 m in elevation.
At the basestation location, the NAD83 ellipsoid height was -11.04 m and the orthometric height based on Geoid09 was 9.16m, representing a vertical offset of 20.2 m between the NAD83 ellipsoid heights and NAVD88. This static offset was applied to the dataset to convert the NAD83 (CORS96) ellipsoid elevations into NAVD88 altitudes. Finally, the ASCII data were gridded in Surfer at 1-m resolution using the IDW algorithm with a 3 m search radius and a smoothing parameter set to 0.25 (the smoothing parameter here accounts for the horizontal uncertainty in the sounding position). This process filled small gaps in the surface and provided some minor smoothing through the statistical noise inherent to interferometric bathymetry. This surface was converted to ESRI ASCII grid format using Surfer's conversion tools.
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Please recognize the U.S. Geological Survey (USGS) as the source of this information. Physical materials are under controlled on-site access. Some USGS information accessed through this means may be preliminary in nature and presented without the approval of the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the responsibility of the user.
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