Coastal Single-Beam Bathymetry Data Collected in 2023 From the Chandeleur Islands, Louisiana

GNSS Acquisition: Two Global Navigational Satellite Systems (GNSS) base stations were established on RFSH and MRK3 benchmarks and were continually occupied and equipped with Spectra Precision SP90M GNSS receivers recording full-carrier-phase positioning signals from satellites at a rate of 0.1 second (s) via Trimble Zephyr 3 Base GNSS antennas. A similar set-up was used on each of the rover vessels except that the GNSS antennas varied by platform (TVEE - Trimble Zephyr 3 Rover; WVR1/WVR2 - Spectra Precision SPGA Rover; and MAKO - Trimble GA830). The maximum processing baseline between the RFSH benchmark and rover vessels was 19.42 km. The maximum processing baseline between the MRK3 benchmark and rover vessels was 21.13 km.

Single-Beam Bathymetry Acquisition: A total of 2,467.64 line-km of SBES data were collected as part of FAN 2023-325-FA: the TVEE collected 915.69 line-km, WVR1 collected 524.58 line-km, the WVR2 collected 523.04 line-km, and the MAKO collected 504.33 line-km of data. Boat motion was recorded on each vessel at 50-millisecond (ms) intervals using a SBG Ellipse Series A motion sensor. HYPACK A Xylem Brand, a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation, steering, correction, data quality parameters, and instrumentation-status to the boat operator. Depth soundings were recorded at 50-ms intervals using an Odom ECHOTRAC CV100 echosounder with a 4-degree, 200-kilohertz (kHz) transducer on the TVEE, WVR1, and WVR2. The MAKO was equipped with a CeeScope echosounder with a 9-degree, 200-kHz transducer. 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 Conductivity, Temperature, and Depth (CTD) instruments. The instruments were periodically cast overboard to record changes in water column speed of sound (SOS). A total of 353 casts recorded sound velocities ranging from 1523.41 to 1550.99 meters per second (m/s) at depths ranging from a minimum of -0.33 m to a maximum of -14.29 m.

Differentially Corrected Navigation Processing: All navigation data were acquired in the World Geodetic System 1984 (WGS84). The latest realization of WGS84 (G2139) was introduced on January 3, 2021. The survey acquisition dates occurred after this date; therefore, it was the realization used for post-processing the navigation data. Base station data were post-processed through NGS OPUS and the coordinate values of each base station were derived from the time-weighted average of values obtained from NGS OPUS. The WGS84 (G2139) coordinates used for post-processing the navigation collected during this survey were 29° 52' 01.11115" North, 88° 50' 02.09576" West, -26.527 m ellipsoid height (RFSH) and 29° 57' 0.54550" North, 88° 49' 34.52742" West, -26.965 m ellipsoid height (MRK3). The rover data, base data, and the base station coordinates were imported into NovAtel's Waypoint GrafNav software. Each kinematic GNSS data session from the survey vessel was post-processed to the concurrent base GNSS data session. Satellite health and data plots, trajectory maps, and processing logs generated by GrafNav provided information that was used to modify processing parameters to attain trajectory solutions (between the base and the rover) that resulted in fixed positions. Some examples include: 1) excluding satellites flagged by the program as having bad health or cycle slips, 2) excluding satellite time segments that introduce positional errors that prevent a fixed solution, or 3) adjusting the satellite elevation mask angle to improve the position solutions. The final differentially corrected, precise DGPS positions were computed at 0.1s and exported in American Standard Code for Information Interchange (ASCII) text format in WGS84 (G2139) Universal Transverse Mercator (UTM) Zone 16 North (16N) geodetic datum.

Bathymetry Processing: All data were processed using HYPACK following steps outlined in the HYPACK 2021 User Manual (Xylem, 2021). First, Geodesy settings were confirmed, ensuring the correct UTM Zone and K-N from User Settings with a user value of 0.00 for Real-Time Kinematic (RTK) Tide were used. Next, data were imported into the Single Beam Editor (64 bit). The following settings were used within the Read Parameters window: on the Survey Tab 'Elevation Mode' was highlighted and the project information was verified. Under the Corrections Tab, the sound velocity profiles (SVP) were added as a VEL file and 'Time and Position' was utilized as the SVP interpolation method. Under the Devices Tab, the vessel offsets were added and saved to a vessel-specific .INI file, ensuring RTK Tides was selected. Under 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; 9 degree for the MAKO) were all selected and applied to each sounding. The data is then processed within the Single Beam Editor (64 bit). Next, the GPS Adjustment tool was used to merge the post-processed navigation solution with their respective HYPACK RAW data files. Outlier soundings were identified through visual inspection and manually removed. Finally, data were smoothed using the smoothing tool with the default setting 'number of sampled for average of 64'. The x,y,z (easting, northing, and ellipsoid height, in meters) data were exported as a text file referenced to WGS84 (G2139) UTM 16N. Each line was exported as a single text file and then merged in R by vessel identifier (BIM06, etc.) to create four individual text files, one for each survey vessel.

Offset Analysis and Correction: During internal quality assurance and quality control (QA/QC) checks on the x,y,z data, it was discovered that differences in survey collection hardware resulted in a vertical offset between the four survey platforms of approximately 20 centimeters (cm), with soundings derived from the Ceescope transducer (MAKO) reading 18.16 cm shallower than soundings derived from the Odom transducers (WVR1, WVR2, TVEE). To accurately identify true changes in the seafloor and not changes associated with hardware differences, an analysis was conducted to quantify the vertical offset between the transducers for each vessel. First, data for each vessel were imported into ArcGIS Pro and the 'XY Table to Point' geoprocessing tool was used to create the point file. Next, the 'Spatial Join' tool was used 6 individual times to join points from a set of two vessels (WVR1-WVR2, WVR1-TVEE, WVR1-MAKO, WVR2-TVEE, WVR2-MAKO, and MAKO-TVEE) using the tool inputs of closest geodesic point within a 1 m search radius. All 6 joins were exported to Excel using the 'Table to Excel' tool. Once in Excel, the offset between each vessel set (A and B) was calculated by averaging the absolute difference in elevation at each joined point. The offsets of the post-processed HYPACK data, prior to adjustment, were as follows: the MAKO and WVR1 had an average offset of 20.55 cm and a root mean squared error (RMSE) of 19.57 cm. The MAKO and WVR2 had an average offset of 20.25 cm and an RMSE of 19.29 cm. The MAKO and the TVEE had an average offset of 13.68 cm and an RMSE of 14.41 cm. When the single-beam data was compared to multibeam data collected during the same survey periods (found in an independent data release), the best agreement was between the Ceescope and the multibeam data. Since the Ceescope transducer started its service in 2023 and the Odom transducers have been in service since 2012, one possible cause of the offset between the Odom transducers and the Ceescope transducer is hardware degradation. To derive an equation to quantify the offsets between vessels, including any biases with depth, elevations from Vessel A were plotted against elevations from Vessel B and a regression analysis was conducted in Excel. The equation from each regression analysis was used to separately adjust the elevations from the WVR1, WVR2, and TVEE shallower and closer to the MAKO data. The equation used to adjust the WVR1 to the MAKO was y = 1.0003x + 0.2073. The equation used to offset the WVR2 to the MAKO was y = 0.9979x + 0.1349. The equation used to offset the TVEE data to the MAKO was y = 0.9887x - 0.263. For all equations y represented the elevation of the MAKO data and x represented the elevation data from WVR1, WVR2, and the TVEE, respectively. The adjustments were calculated in MATLAB and all offset data was appended to the files in this data release as a new column, retaining the original elevation data column. The final offsets, after adjusting the elevations in MATLAB were as follows: the MAKO and WVR1 were offset by an average of 6.39 cm with a RMSE of 15.22 cm. The MAKO and WVR2 were offset by an average of 4.10 cm with a RMSE of 5.38 cm. The MAKO and the TVEE were offset by an average of 4.59 cm with a RMSE of 7.02 cm

Uncertainty Analysis: The MAKO data exported from HYPACK and the adjusted elevation x,y,z .txt files from MATLAB for WVR1, WVR2, and the TVEE, as well as one file with all data combined were opened in Esri ArcGIS Pro as points utilizing the 'XY Table to point' geoprocessing tool. The projection was set to Geographic WGS84 UTM 16N. The generated points were visually reviewed for any obvious outliers or problems. Next, polylines (representing tracklines) were produced from the point shapefiles using the 'Points to Line' geoprocessing tool for each survey platform (MAKO, WVR1, WVR2, and TVEE). Utilizing both the points (x,y,z) and tracklines (polyline), a Python script in ArcGIS Pro was used to evaluate elevation differences at the intersection of crossing tracklines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation with a search radius of 3 m. The crossing analysis yielded a RMSE of 7.44 cm for all crossings. When the TVEE crossed one of its own lines, the crossing analysis yielded a RMSE of 5.97 cm. When the WVR2 crossed one of its own lines, the crossing analysis yielded a RMSE of 4.59 cm. When the WVR1 crossed one of its own lines, the crossing analysis yielded a RMSE of 4.78 cm. When the MAKO crossed one of its own lines, the crossing analysis yielded a RMSE of 5.65 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 datums (z) from WGS84 (G2139) UTM 16N (horizontal), WGS84 (G2139) ellipsoid height (vertical) to NAD83 UTM 16N (horizontal), NAVD88 orthometric height (vertical) using GEOID18. The VDatum-reported vertical uncertainty for the file Chandeleur_Islands_2023_SBES_NAD83_UTM16N_NAVD88_G18_LTC_xyz.txt is 0.066 m, and the vertical uncertainty for the file Chandeleur_Islands_2023_SBES_NAD83_UTM16N_NAVD88_G18_NoLTC_xyz.txt is 0.065 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.