Single-Beam Bathymetry Data Collected in March 2021 from Grand Bay and Point Aux Chenes Bay, Mississippi/Alabama

GPS Acquisition: Two Geographic Positioning System (GPS) base stations were established throughout the survey area, both of which were located on NPS established benchmarks. Benchmark 189A (PID: D05977) was located to the north-north west of the survey area near the railroad tracks at the entrance of the Grand Bay National Estuarine Research Reserve (GNDNERR), B166 (PID: D05987) was located northwest of the survey area at the boat launch location used for this survey. The base stations were continually occupied and equipped with a Spectra Precision SP90M (B166) and a Spectra Precision Proflex 800 (189A) GPS receiver recording full-carrier-phase positioning signals (L1/L2) from satellites via Thales Choke-ring antennas, recording at a rate of 0.1 s.

Single-Beam Bathymetry Acquisition: The single-beam bathymetric data were collected under the USGS Field Activity Number (FAN) 2021-307-FA, which encompass data from two separate survey platforms; the RV Shark (WVR1, 21CCT01), a 12-ft (foot) Yamaha PWC which collected 28.51 line-km (97 lines), and the RV Chum (WVR2, 21CCT02), an additional 12-ft Yamaha PWC which collected 190.28 line-km (91 lines). Boat motion was recorded at 50-millisecond (ms) intervals using a SBG Ellipse A #1 motion sensor aboard each PWC. HYPACK (version 18.1.8.0), a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation, steering, correction, data quality, and instrumentation-status information to the boat operator. Depth soundings were recorded at 50-ms intervals using an Odom echotrac CV100 sounder with a 200-kilohertz (kHz) transducer. Data from the GPS receiver, motion sensor, and fathometer were recorded in real-time aboard all vessels independently 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 three SonTek Castaway Conductivity, Temperature, and Depth (CTD) instruments. The instruments were periodically cast overboard to observe changes in water column speed of sound (SOS). A total of 74 successful sound velocity casts were taken throughout the survey at an average depth of 0.94 meters, and on average produced a sound velocity of 1492.32 meters per second (m/s).

Differentially Corrected Navigation Processing: The coordinate values of the GPS base stations are the time-weighted average of values obtained from NGS OPUS. The base station coordinates were imported into GrafNav version 8.7 (Waypoint Product Group) and the kinematic GPS data from the survey vessel were post-processed to the concurrent GPS session data at the base stations. During processing, steps were taken to ensure that the trajectories between the base and the rover were free of erroneous solutions and resulted in fixed positions. By analyzing the graphs, trajectory maps, and processing logs that GrafNav produces for each GPS session, GPS data from satellites flagged by the program as having poor health or satellite time segments that had cycle slips could be excluded, or the satellite elevation mask angle could be adjusted to improve the position solutions. The final differentially corrected, precise DGPS positions were computed at 0.1 s and exported in American Standard Code for Information Interchange (ASCII) text format. Concurrent post-processed navigation data to single-beam data points replace the uncorrected rover positions, recorded during acquisition, in subsequent processing steps. The GPS data were processed and exported in the World Geodetic System of 1984 (WGS84) G1762 geodetic datum.

All data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 11.3.0. The raw HYPACK data files were imported into CARIS, the differentially corrected navigation files were imported using the generic data parser tool, and any SVP casts were entered and edited using the SVP editor. The bathymetric data components (position, motion, depth, and SOS) were then merged and geometrically corrected in CARIS to produce processed x,y,z data. Next, the data were edited for outliers and then further reviewed in the Subset Editor utility for crossing status, and questionable data points or areas. The geometrically corrected point data were then exported as an x,y,z ASCII text file referenced to WGS84 (G1762), ellipsoid height in meters.

Quality Control, Quality Assurance (QA/QC) and Uncertainty Analysis: All single-beam data exported from CARIS and elevation data exported from GrafNav, were imported into Esri ArcMap version 10.6.0 utilizing the create feature class from an XY table function and plotted in 0.25-m color coded intervals utilizing symbology functions. First, all data were visually scanned for any obvious outliers or problems. Next, a trackline shapefile was produced using X-tools Pro (version 21.1.4313) "Make Polylines from Points" function for each survey platform. Utilizing both the x,y,z (point) and trackline (polyline) shapefiles, a Python script evaluated 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 1 m. The RMS error for WVR1 (21CCT01), when crossing a trackline it previously surveyed, was 5.06 cm and when WVR2 (21CCT02) crossed a track line it previously surveyed, the RMS error was 4.60 cm. Once individual platform statistics were obtained, all data were merged, and crossing analysis yielded a 5.32 cm RMS error. These merged files were separated into three survey areas: Point Aux Chenes, Point Aux Chenes Reefs and Middle Bay. These files were then exported from Esri ArcMap as x,y,z text (.txt) files using the X-tools Pro "table to text" function, and made available in the download section of this data release, along with the populated trackline shapefiles.

Datum Transformation: NOAA's VDatum v.4.1 was used to transform the single-beam data points (x,y,z data) from their data acquisition datum (WGS84 G1762) to NAD83 NAVD88 using the NGS geoid model of 2012A (GEOID12A). For conversion from the WGS84 ellipsoid to NAVD88, there is a total of 7.616 cm of uncertainty in the transformation (NOAA/NGS's VDatum: Estimation of Vertical Uncertainties in VDatum, https://vdatum.noaa.gov/docs/est_uncertainties.html). Resultant data files have been made available in the downloads section of this data release.

Shoreline Preparation: In order to provide the most accurate representation of the bathymetry within the nearshore areas, shoreline data was prepared and used in conjunction with the x,y,z data during subsequent processing steps. Shoreline polyline data provided in Terrano and others (2021), was first converted to points utilizing the XTools Function "Polylines to Points". Utilizing the "Buffer" analysis tool, a 100 m buffer was generated around the Point Aux Chenes and Middle Bay tracklines. The shoreline points were then clipped using the "Clip" analysis tool to each generated buffer file to ensure only shoreline points that fall within 100 m from a bathymetry point were utilized for further processing. Some manual editing of the resultant shoreline point file was necessary to ensure proper representation of the bathymetry extent. An integer field was added to the shoreline points attribute table named, "NAVD88_G12A" and the field populated with a zero-value utilizing the Field Calculator function to represent a zero elevation at the shoreline extent. The resultant point shapefile has been made available in the download section of this data release (Stalk and others, 2021). The shoreline points were then converted to a polygon utilizing the XTools Pro "Points to Polygons" function to establish an extent file to be used in later processing steps. Polygon vertices were manually added using the Editor tool to the western and eastern portions of Point Aux Chenes Bay, placed at the approximate land-water intersection using Esri's Imagery base map, as well as along the offshore extent of the x,y,z data. The resultant extent or coverage file was utilized in subsequent processing steps and has also been made available in the data download section of this data release (Stalk and others, 2021).

Create Digital Elevation Model: The transformed x,y,z data described in the previous processing step were imported into ArcMap using the "Create Feature Class From XY Table" tool. The dataset, along with the generated shoreline points were used to create a triangulated irregular network (TIN) using the "Create TIN" tool. The TIN was subsequently converted into a raster file using the "TIN to Raster" tool with a cell size of 10 meters for both the Point Aux Chenes Bay and Middle Bay survey areas. The resultant raster was then clipped utilizing the "Extract by Mask" Analysis tool, utilizing the coverage file produced in the previous step. The final digital elevation model (DEM) was then exported from ArcMap as a 32-bit floating GeoTiff, utilizing the Natural Neighbor algorithm, and made available within the data downloads section of this data release (Stalk and others, 2021). As a part of standard QA/QC procedures, each DEM was compared to the associated x,y,z data, and associated RMSe values were populated. The Point Aux Chenes Bay DEM has a RMSe value of 0.027 m and the Middle Bay DEM, 0.036 m respectively.