SWATHplus acquisition software (version 3.05.90) was used to digitally log the bathymetric data at a rate of 30 pings/second and 3,072 samples per swath (ping) in the SWATHplus SXR format. Data collection parameters are saved into a SWATHplus session file in SEA's SXS format. These files can be later used for data replay.
An Octopus F180R Attitude and Positioning system (see: <http://www.codaoctopus.com/motion/f180/index.asp>) recorded ship motion (heave, pitch, roll, and yaw). These data were transmitted via network connection to the SWATHplus data collection software. The Octopus F180R Inertial Measurement Unit (IMU) was mounted directly above the SWATHplus transducers, to minimize lever arm offsets that can lead to positioning errors. The F180R uses two L1 antennas for position and heading accuracy. The antennas are mounted on a rigid horizontal pole, 3 meters above the F180R IMU, with a horizontal separation of 1 meter and are offset from the IMU in a forward/aft configuration. The forward offset of the primary antenna from the IMU is 0.5 meters, with no port/starboard offset.
Eight sound-velocity profiles were acquired during survey operations at roughly 4-hr intervals using an Applied Microsystems SV Plus V2 Velocimeter (Applied Microsystems, 2008).
Offsets of the SWATHplus transducers and DGPS navigation antenna from the Octopus F180R Attitude and Positioning System Inertial Measurement Unit (IMU), which is the designated common reference point (CRP), were verified as entered correctly into the SWATHplus session file for the survey. Offsets forward, below, and to the right (starboard) of the CRP are considered positive:
Primary navigation antenna to IMU: Height offset: -3 meters, Forward offset: 0 meters, Starboard offset: 0 meters
Draft of the MRU below water line: 0.63 meters.
Offset of SWATHplus transducer 1 (port side) from the CRP: Height offset: 0.158 meters Forward offset: -0.019 meters Starboard offset: -0.11 meters
Offset of SWATHplus transducer 2 (starboard side) from the CRP: Height offset: 0.158 meters Forward offset: -0.019 meters Starboard offset: 0.11 meters
Software: SWATHplus 3.05.19.0
Data from eight sound velocity profiles were incorporated into the SWATHplus session file. The velocity data is stored in Microsoft Office Excel 2003 SP3 commas-separated values (csv) format, and the location and time, as well as the velocity information, were extracted from the csv file and entered into the sound velocity profile dialog in SWATHplus. Sound velocity information closest in time to the bathymetric data collected are utilized by the SWATHplus program to correct for variations in sound velocity through the water column, minimizing ray bending effects that can produce erroneous depth soundings.
Software: SWATHplus 3.05.19.0
Various filters were set in the SWATHplus software in order to refine the depth of the seafloor across the swath for each ping based on the phase and amplitude of the returning sonar signal. Only the filters listed below were set, all others were turned off. Filter settings:
Low amp: 100; Range: Max amplitude 1 meter, min amplitude 0 meters; Phase Confidence: 70 percent; Angle Proximity: 25 sample window, range 0.11, threshold 9 samples, Minimum elevation -120 degrees; Box: Minimum Depth 0 meters, Minimum Horizontal Range 1 meter; Median sample window size 5; Along track 1: Max depth difference 10 meters, window size 5, learn rate 0.6 Along track 2: Max depth difference 5 meters, window size 1, learn rate 0.9; Mean output processed pixel size: 0.2 meters.
Software: SWATHplus 3.05.19.0
Patch test, used to calibrate roll offsets, lines were replayed through the SWATHplus software after the velocity profiles (for that day) and filters were set and saved in the SWATHplus session file. Processed data files were saved in the SWATHplus sxp format.
The patch test processed sxp files (p1 p8.sxp) were imported into the SWATHplus Grid Processor program and run 2 times through the calibration tool to determine any static offsets for roll. Results: Port roll offset = +0.254 degrees, Stbd roll offset = -1.037 degrees. These offsets were then entered into the SWATHplus session file.
Software: SWATHplus Grid Processor 3.05.19.0
The Ashtech Z-Xtreme receiver collected position (x,y) and elevation (z) data of the DGPS antenna during survey operations (see: Source Contribution for antenna configuration). DGPS position data (xyz) were recorded with HYPACK Hydrographic Survey Software (<http://www.hypack.com/>). Real-Time Kinematic (RTK) corrections based on vertical position data from the Fort Gratiot, Michigan National Geodetic Survey Continuously Operating Reference Stations (CORS) were applied to the recorded antenna heights (z) during post-processing using NovAtels Waypoint GrafNav post-processing high-precision package, a static kinematic/baseline processor (<http://www.novatel.com/>). A rover RTK-GPS station was established at the tidal benchmark at the U.S. Coast Guard Base at Port Huron in order to determine the offset between North American Vertical Datum of 1988 (NAVD 88) (vertical datum referenced at the CORS site) and the International Great Lakes Datum 1985 (IGLD 85) referenced at the tidal benchmark.
An offset was applied to the RTK-corrected antenna heights to convert the reference heights to the local chart datum, International Great Lakes Datum 198 (IGLD85). Additional offsets were applied to translate the height of the antenna to the waterline. The resulting data file contained date, time (every second) and height of waterline relative to IGLD85.
(Navigation data are stored in American Standard Code for Information Interchange (ASCII) files. Offsets were applied using GNUs Not Unix (GNU) gawk 3.1.5 programming language).
NovAtel GrafNav 8.1.0; GNU gawk 3.1.5; HYPACK 8.2.3.7
BEGIN { latmin = 0.0}{min= substr($1,4,2); if(lastmin !=min) {printf("%s %s %.2f\n", $1, $2, $3)} lastmin=min;}
The filtered elevations at one-minute time intervals were then entered into SEA, LTD SWATHplus software as a Tide in the following format: HR:MN DD/MM/YYYY XXX.XX, where HR=hour, MN=minute, DD=day, MM= month, YYYY=year, XXX.XX=tide.
MATLAB 7.4.0 (R2007a)
Software: SWATHplus 3.05.19.0
The SXP files were then edited using the following filters: across track distance 7 times nadir depth (reduces the swath width to seven times the water depth in order to reduce noise in the outer range), across track angle of 2 degrees (calculates the slope of each beam in degrees to the prior and post beams within the same swath. If slopes exceed the specified value and are of opposite sign the beam is rejected), Missing neighbors: port and starboard, forward and aft, and any 2 of 4 (filters the soundings if neighboring soundings to the port and starboard, forward and aft, or if two of the four neighboring beams are missing) (<http://www.caris.com>). Individual files were further edited with the SWATH EDITOR where remaining outliers were manually removed.
Field sheets were then created with UTM zone 17N, WGS84 projection and used to organize the processed bathymetric data and generate A Bathymetry Associated with Statistical Error (BASE) surfaces. Three BASE surfaces were created at the following resolutions: 0.5, 2.0 and 5.0 meters and the swath angle Surface Type (default). The maximum footprint size in the BASE surface that a sounding was utilized was set to 9 pixels, and the include status for soundings were set to accepted, examined, and outstanding.
Each of the BASE surfaces was interpolated in CARIS HIPS. The interpolation is only applied to areas of no data, and helped to fill in small gaps in the surface. Parameters used for the interpolation were: Matrix size 5X5 pixels, minimum number of neighboring pixels required for interpolation was 6.
Software: CARIS HIPS/SIPS 6.1
The 0.5 m bathymetric grid contained data gaps at nadir. The filters used within the SWATHplus software eliminated noisy data near nadir, resulting in data gaps of less than 5 meters at nadir. To fill these data gaps, the IVS command dtmmerge from within the Fledermaus Commander was used to merge the high-resolution 0.5 meter grid with the lower resolution 2.0 grid.
Dtmmerge in file1_highres.dtm file2_lowres.dtm out output.dtm cellsize 0.5
By default, the lower resolution grid is used to fill data gaps present within the higher resolution grid, with the output grid reflecting the high-resolution input grid: 0.5 meters.
After merging the 0.5- and 2.0-meter grids, several data gaps remained in areas where adjacent tracklines did not provide complete coverage of the riverbed. Dtmmerge was run again using the output from the first pass (merging the 0.5 and 2 meters grids) and the 5-m resolution grid in order to minimize data gaps between lines.
The final grid, bathy_05m, was exported from IVS Data Magician as an Arcview Grid (ESRI ASCII grid) (bathy_05m_m.asc).
Software: IVS DMagic 6.7.0
The projection of the grids was then defined as UTM, zone 17N, WGS84, using Arc Toolbox, Data Management Tools, Projections and Transformations, Define Projection.
In order to reduce spurious bathymetric points along the perimeter of the bathymetric grid, the border of the grid was digitized and saved as a polygon shapefile (border.shp). The Spatial Analyst Tool Extraction Extract by Mask was then used to clip the bathymetric grid (bathy_05m_m) based on the digitized border (border.shp), thus eliminating spurious points at the edges of the bathymetric grid. The output clipped grid was stored as bathy_05m.
Software: ESRI ArcGIS 9.2
In order to smooth the contours, an analysis cell size of 50 meters was chosen (Spatial Analyst, Options, Cell Size). High frequency undulations were still present in the contour data generated from bathy_05m. Mville_05m and portl_05m contained a small number of contours due to the small size of the survey area and relatively smooth bathymetry. These areas did not require additional smoothing. To minimize this "chatter" the Smooth Line tool within ArcToolbox Generalization was used with a smoothing tolerance of 50 meters.
The 'Merge' tool was then used in ArcToolBox, Data Management Tools, General, Merge, to merge the contours generated from bathy_05, mville_05m, and portl_05m bathymetric grids. The output filename was stored as 'con_05m'.
The polyline shapefile was then reprojected in Geographic Coordinate System, WGS84, using the ArcToolbox Projections and Transformations, Feature, Project tool.
ArcGIS 9.2
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