Bathymetric data, stored as elevation above IGLD85, collected by the U.S. Geological Survey within the St. Clair River offshore of Marysville, Michigan, 2008 (ESRI GRID, MVILLE_05M)

SWATHplus Configuration
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

Sound Velocity Profiles
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

Bathymetric Filters
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: Grid Processor 3.05.19

Post-Processed Navigation (RTK corrections applied to DGPS data):
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

The date, time (every second) and height of waterline relative to IGLD85 were imported to The MathWorks MATLAB technical computing software (version 7.4.0 (R2007a)). A median filter (medfilt1) using a 361 boxcar (approximately a 6 minute filter) was applied to smooth the chatter present in the elevation data. The filtered elevations were then exported from MATLAB. A GNU gawk script was used to downsample the output file from every second to every minute:
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)

Raw SXR bathymetry data were replayed line by line through the SEA SWATHplus software to generate processed SWATHplus sxp files. The following information was stored within the SWATHplus session files and applied to the raw bathymetric soundings: eight sound velocity profiles, TIDE (RTK-corrected water level heights relative to IGLD85), roll offsets calculated during pre-survey patch test, draft of the transducer below the water line, and the measured angles and relative positions of the swath bathymetric transducers. The SWATHplus software applies the pitch, roll, heave, and heading data supplied by the F180R IMU, to compensate depth solutions across the swath for any ship motion.
Software: SWATHplus 3.05.19.0

All the processed SWATHplus sxp files were imported into CARIS HIPS/SIPS Hydrographic Data Processing System software (www.caris.com) using the CARIS HIPS Conversion Wizard (specifying the SWATHplus format, RawData (sxp files), a CARIS HIPS project, vessel, and julian day, ground coordinates [UTM zone 17N WGS84] and speed of sound of 1450 meters/second as measured at the SWATHplus transducer face). The conversion wizard converts depth, navigation, and motion data recorded in the sxp files into an internal CARIS HIPS format.
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. Individual field sheets and BASE surfaces were generated for the Upper St. Clair, Marysville and Port Lambton study areas.
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

Interactive Visualization Systems (IVS) Data Magician software (version 6.7.0k) was used to import the CARIS BASE surfaces. These data were imported as Gridded Data in the CARIS Base Surface File Type. The CARIS Base Surfaces were converted and saved in IVS dtm and geo format.
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.
The final grid was exported from IVS Data Magician as an Arcview Grid (ESRI ASCII grid).
Software: IVS DMagic 6.7.0

The ESRI ASCII grid was then imported to ArcGIS 9.2 using the ASCII to Raster Tool within ArcToolbox - Conversion Tools - To Raster.
The projection of the grid was then defined as UTM, zone 17N, WGS84, using Arc Toolbox, Data Management Tools, Projections and Transformations, Define Projection.
Software: ESRI ArcGIS 9.2

The bathymetric grid was converted from RASTER to ASCII using ArcGIS 9.2, ArcToolbox, Conversion Tools, From Raster, Raster to ASCII.
ArcGIS 9.2

Edits to the metadata were made to fix any errors that MP v 2.9.36 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). The link to the data in the Distribution_Information section had to be fixed. Attempted to modify http to https where appropriate. Reordered the links in the identification section to have a landing page link as the first link. The source information was incomplete and had to be modified to meet the standard. The distribution format name was modified in an attempt to be more consistent with other metadata files of the same data format. Added the AAIGrid distribution formation to account for the ASCII grid present in the zip file. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.