Coastal Single-beam Bathymetry Data Collected in 2024 From Breton Island, Louisiana

GNSS Acquisition: BRET benchmark located on Breton Island, Louisiana was used for local navigation control. It was equipped with a Spectra Precision SP90M GNSS receiver with a Trimble Zephyr 3 Base GNSS antenna recording full-carrier-phase positioning signals from satellites at a rate of 0.1 second (s) (equal to 10 hertz [Hz]). The same GNSS receivers were used on the rovers, except the GNSS antennas varied by vessel (TVEE - Trimble® Zephyr 3 Rover; WVR1/WVR2 - Spectra Precision® SPGA Rover). The base station GNSS data files were converted from Spectra Precision proprietary G-file formats (e.g. GBRETA24.218) into the non-proprietary Receiver Independent Exchange Format (RINEX [https://igs.org/wg/Rinex/]) version 2.11 (e.g. BRET218A.24O) for submission to NOAA NGS OPUS. In preparation for post processing at a later step, using the RINEX converter again, both the base and rover G-files were converted into RINEX version 3.04 (e.g. TVEE00000_R_20242182001_02H_10Z_MO) which converts all the satellite constellations tracked by the antenna (GPS, GLONASS, Bideau, and Galileo). An example of the RINEX 2.11 filename is TVEE219A.24O where the convention TVEE is the four-letter site name, 219 is the day of year, A is the session A-Z, 24 is the year, O represents the observation file, N is the navigation file, M is meteorological file, and G is the GLONASS navigation message file. An example of the RINEX 3.04 filename is TVEE00000_R_20242182001_02H_10Z_MO where the convention TVEE00000 is the nine-letter site name, R is the data source (R - receiver), 20242182001 is the year (2024), day of year (218), and UTC start time of the file in HHMM (2001), 02H is the file time (2 hours), and the 10Z is the recording rate (10 Hz), MO is the mixed observation file, MN is the mixed navigation file, and MM is meteorological file.

Single-Beam Bathymetry Acquisition: A total of 583 line-km of SBES data were collected as part of FAN 2024-320-FA: the R/V G. Hill (TVEE - 24BIM07) collected 308.60 line-km, R/V Shark (WVR1- 24BIM08) collected 151.82 line-km, and R/V Chum (WVR2-24BIM09) collected 122.47 line-km of data. Boat motion (heave, roll, pitch) was recorded on each vessel at 50-millisecond (ms) intervals using an SBG Ellipse Series A motion sensors. HYPACK®, a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation parameters, course corrections, data quality parameters, and instrumentation-status to the boat operator. Depth soundings were recorded at 50-ms intervals using a Teledyne Odom ECHOTRAC CV100 echosounder with a 4-degree, 200-kilohertz (kHz) transducer on the TVEE, WVR1, and WVR2. For each vessel, data from the GNSS receiver, motion sensor, and echosounder were recorded in real-time and merged into a single raw data file (.RAW) in HYPACK®, with each device string referenced by a device identification code and time stamped to Coordinated Universal Time (UTC). Sound velocity profile (SVP) measurements were collected using SonTek Castaway-CTD® (Conductivity, Temperature, and Depth) instruments. The CTD units were cast overboard to record changes in water column speed of sound (SOS) spatially and temporally. A total of 312 casts recorded sound velocities ranging from 1522.24 to 1545.26 meters per second (m/s) at water depths ranging from a minimum of -0.15 m to a maximum of -10.80 m.

Differentially Corrected Navigation Processing: All navigation data were acquired in WGS84 (G2296)/International Reference frame 2020 (ITRF2020) which was introduced on January 7, 2024. The survey acquisition dates occurred after this date. The latest processing software program versions were limited to WGS84 (G2139); therefore, it was the realization used for post-processing the navigation data. The base station data were post-processed through NOAA NGS OPUS, and the coordinate values were derived from the time-weighted average of values produced by OPUS. The 2024 GNSS base occupations were within +/- 3 standard deviations of the 2022-time weighted average coordinates, therefore the 2022 WGS84 (G2139) coordinates from Lyons and others (2024) were able to be used for post-processing the navigation data collected during this survey. The 2022 WGS84 (G2139) coordinates used were 29° 29' 38.33200" North, 89° 10' 29.28105" West, -25.253 m ellipsoid height, and epoch 2022.590 (BRET). To check the difference between realizations (G2139 and G2296), the BRET base station coordinates were sent through HTDP online and the resultant difference was 0.00006 latitude seconds, 0.00005 longitude seconds, and 0.00 meters ellipsoid height. The rover data, base data, and the base station coordinates were imported into GrafNav, NovAtel's Waypoint software (NovAtel, 2023). Each kinematic GNSS data session from the survey vessel was post-processed to the concurrent static base GNSS data session. GNSS data plots and data information generated by GrafNav including satellite health (satellite lock, satellite cycle slips), rover trajectory maps, and pre-processing logs were used to modify the processing parameters to attain trajectory solutions (between the base and the rover) that resulted in fixed positions. Primary examples include: 1) excluding satellites, flagged by GrafNav, with bad health or frequent cycle slips, 2) omitting satellite time segments that introduce positional errors which prevented a fixed solution, or 3) adjusting the satellite elevation mask angle to improve the position solutions (NovAtel, 2023). The final differentially corrected, precise DGPS positions were computed at 0.1s and exported as single data point epochs in ASCII text format in WGS84 (G2139) UTM 16N geodetic datum.

Single-beam Bathymetry Processing: All data were processed using HYPACK® 2023 following steps outlined in the HYPACK® 2023 User Manual (Xylem, 2023). First, the HYPACK® project geodesy settings were verified, ensuring the correct UTM Zone and "K-N from User" settings with "height of ellipsoid above chart datum" value of '0.00' were selected. Next, the .RAW data files were imported into HYPACK® Single Beam Editor (64 bit). The following "Read Parameters" were selected within the Survey Tab 'Elevation Mode' and the project information was verified. Within the Corrections Tab: the sound velocity profiles (SVP) were added as .VEL format, and 'Time and Position' was selected for the SVP interpolation method. Within the Devices Tab: the vessel offsets were added and the RTK Tides box was selected. Within the Processing Tab: 'Apply Heave Correction', 'Correct Induced Heave', 'Remove Heave Drift', 'Avoid Double Heave', 'Merge Tide Data with Heave', 'Interpolate Draft', 'Apply Pitch and Roll Corrections', and 'Steer Sounding Beam' (4 degree for the TVEE, WVR1, and WVR2) were all selected. The data were then processed within HYPACK®'s Single Beam Editor (64 bit). Next, the GPS Adjustment tool was used to merge the post-processed navigation trajectory epoch solutions with their respective HYPACK® data files (.RAW). Using the Editor Profile window, these processed soundings were visually scanned for any resultant outliers which were manually removed using the editor tool. The data were then smoothed using the smoothing option with the 'number of samples for average" set to 60. Finally, the x,y,z data (easting, northing, ellipsoid height) and associated attributes (calendar date, UTC time, and line number) were exported as a single text file per line. The text files were then combined into one file per subFAN/vessel using the ArcGIS Pro 'Merge (Data Management)' geoprocessing tool, resulting in three main text files.

Quality Control, Quality Assurance (QA/QC) and Uncertainty Analysis: Using ArcGIS Pro, the processed single-beam datasets were then transformed into feature classes using the "Create Points From Table > XY Table to Point" geoprocessing tool and the spatial reference was set to WGS84 UTM 16N. Next, a polyline feature class (representing tracklines) was produced from the point data using XTools Pro "Make Polylines from Points" (the selection criteria being HYPACK® line name and then ordering by UTC time). Utilizing both the point and polyline feature classes, an ArcGIS Pro Python script was used to evaluate the elevation differences at the intersections of tracklines. The script does this by calculating the elevation difference between points at each intersection using an inverse distance weighting equation with a search radius of 1 m. The crossing analysis yielded a Root Mean Square Error (RMSE) of 5.44 cm for all crossings. When the TVEE crossed one of its own lines, the crossing analysis yielded an RMSE of 4.11 cm. When the WVR1 crossed one of its own lines, the crossing analysis yielded a RMSE of 4.20 cm. When the WVR2 crossed one of its own lines, the crossing analysis yielded a RMSE of 4.14 cm. When the TVEE crossed WVR1, the crossing analysis yielded and RMSE of 8.10 cm. When the TVEE crossed WVR2, the crossing analysis yielded and RMSE of 6.5 cm. When WVR1 crossed WVR2, the crossing analysis yielded an RMSE of 6.00 cm. The crossings in ArcGIS Pro were solely used as a QA/QC, not as a means of editing the data.

Datum Transformation: NOAA NGS's VDatum software was used to transform the single-beam data points' horizontal (x,y) and vertical (z) datums from WGS84 (G2139) UTM 16N (horizontal), WGS84 (G2139) ellipsoid height (vertical) using epoch year 2024.596 (start of the survey) to NAD83 UTM 16N (horizontal), NAVD88 orthometric height (vertical) using GEOID18 epoch year 2010.000 (the reference epoch for the NAD83[2011] realization). The VDatum-reported vertical uncertainty is 0.045 m. For more information about the positional accuracy for these datum transformations, visit the Estimation of Vertical Uncertainties VDatum webpage, https://vdatum.noaa.gov/docs/est_uncertainties.html.