Coastal Single-Beam Bathymetry and Beach Elevation Data Collected in 2024 From Wallops and Assawoman Islands, Virginia

GNSS Acquisition: Two Global Navigational Satellite Systems (GNSS) base stations were established on WFF06 and G421 benchmarks; each base station was continually occupied and equipped with a Spectra Precision SP90M GNSS receiver recording full-carrier-phase positioning signals from satellites at a rate of 0.1 second (s) via a Trimble Zephyr 3 Base GNSS antenna. A similar set-up was used on each of the rover vessels except that the GNSS antennas varied by platform (PWCs – SP90M GNSS receiver Spectra Precision SPGA Rover antenna; beach elevation surveys – Spectra Precision SP-80 or SP-85 integrated GNSS receiver and antenna mounted on a UTV or GPS backpack). The maximum SBES processing baseline between the WFF06 benchmark and rover vessels was 10.23 kilometers (km). The maximum SBES processing baseline between the G421 benchmark and rover vessels was 13.90 km. The maximum beach processing baseline between the WFF06 benchmark and rover vessels was 7.33 km. The maximum beach processing baseline between the G421 benchmark and rover vessels was 11.11 km.

Single-Beam Bathymetry Acquisition: A total of 544.60 line-km of SBES data were collected as part of FAN 2024-310-FA: the R/V Shark collected 275.59 line-km and the R/V Chum collected 269.01 line-km. Boat motion was recorded on each vessel at 50-millisecond (ms) intervals using a SBG Ellipse 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, 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 R/V Shark and the R/V Chum. 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 43 successful sound velocity casts were collected and ranged in depth from 0.14 to 9.63 m, and in sound velocity from 1498.47 to 1522.07 meters per second (m/s).

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 01/03/21. The survey acquisition dates followed 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 was derived from the time-weighted average of values obtained from NGS OPUS. The NGS published elevation for base station G421, which was last determined by differential leveling in 1991, was more than 3 standard deviations outside of the OPUS-derived time-weighted average elevation; therefore, the coordinate values derived from this survey were used for post-processing. Similarly, because there are no published coordinates for base station WFF06, the time-weighted average coordinate values derived from this survey were used for post-processing. The WGS84 (G2139) coordinates used for post-processing the navigation collected during this survey were 37° 51' 51.95987" North, 75° 30' 26.38946" West, -32.969 m ellipsoid height (G421) and 37° 51' 45.73588" North, 75° 30' 33.10669" West, -32.545 m ellipsoid height (WFF06). The kinematic trajectories (rover to base) and base station coordinates were imported to 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 and data plots, trajectory maps, and processing logs that GrafNav produces provide information that can be used to modify processing parameters to attain trajectory solutions (between the base and the rover) free of erroneous data that result in fixed positions. Some examples include 1) excluding satellites flagged by the program as having poor 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 (NovAtel, 2020). The final differentially corrected, precise DGPS positions were computed at 0.1s and 0.5s for SBES and beach surveys, respectively, and exported in American Standard Code for Information Interchange (ASCII) text format in WGS84 (G2139) UTM Zone 18 North (18N) geodetic datum. These processing steps were repeated for all navigation data for both the single beam and the beach elevation datasets.

Bathymetry Processing: All data were processed in HYPACK using the 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 processed in the Single Beam Editor (64 bit). The following settings were used within the Read Parameters window: the survey tab highlighted ‘Elevation Mode’ and was filled with project information. 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’ were all selected and applied to each sounding. Next, the GPS Adjustment tool was used to incorporate the post-processed navigation files. Outlier soundings, identified as distant from otherwise coherent and consistent soundings, were manually removed, and the data were smoothed using the smoothing tool with the default settings. The xyz (easting, northing, and ellipsoid height, in meters) data were exported as a text file referenced to WGS84 (G2139) UTM zone 18N. Each line was exported as a single text file and then merged in R-Studio (2024.04.02 Build 764), a computing software, by vessel identifier (24BIM01, etc.) to create one individual text file for each survey vessel.

Offset Analysis and Correction: Internal quality assurance and quality control (QA/QC) of the SBES xyz data identified a vertical offset between the two SBES datasets and concurrently collected multibeam data (Bemelmans and others, 2025) of 27 centimeters (cm), with the MBES data sitting shallower. The vertical offset between the SBES and MBES surveys is consistent with offset findings from Lyons and others (2025) for the same vessels, confirming the need to adjust this dataset. To accurately identify true changes in the seafloor and not changes associated with hardware differences, vertical offsets were quantified 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 three individual times to join points from a set of two vessels (WVR1-WVR2, WVR1-MBES, and WVR2-MBES) using the tool inputs of closest geodesic point within a 0.5 m (MBES-WVR1/WVR2) or 1 m (WVR1-WVR2) search radius. All 3 joins were exported to Excel using the ‘Table to Excel’ tool. The offset between each vessel set (A and B) was calculated in Excel 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 MBES and WVR1 SBES had an average offset of 27.25 cm and a root mean squared error (RMSE) of 27.73 cm. The MBES and WVR2 SBES had an average offset of 27.18 cm and an RMSE of 27.52 cm. The WVR1 and WVR2 SBES had an average offset of 5.35 cm and an RMSE of 9.32 cm. Due to offsets between SBES and MBES sensors that exceeded system errors, linear regression was used to compare SBES and MBES soundings. SBES data were adjusted using y = 0.9848x - 0.942 for WVR1 and y = 0.9967x - 0.4175 for WVR2. For each equation, x represents MBES elevations and y represents SBES elevations. The adjustments were calculated in R-Studio, a computing software, and all adjusted data were appended to the files in this data release as a new attribute (_adjusted), retaining the original elevation data column.

Uncertainty Analysis: The SBES data was exported from HYPACK (xyz ASCII text file) and merged into one file for each vessel using a script in R-Studio, a computing software. Each vessel .txt file was imported into Esri ArcGIS Pro as a point shapefile (.shp) using the "XY Table to Point" geoprocessing tool. The horizontal datum projection was set to WGS84 UTM 18N. The shapefiles were visually reviewed for any obvious outliers or problems. Next, polyline shapefiles (representing tracklines) were produced from the point shapefiles using the "Points to Line" geoprocessing tool for each survey platform (subFANs 24BIM01 and 24BIM02). Utilizing both the xyz (point) and trackline (polyline) shapefiles, 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 7.99 cm RMSE for all crossings. When the R/V Chum crossed one of its own lines, the crossing analysis yielded a 4.28 cm RMSE. When the R/V Shark crossed one of its own lines, the crossing analysis yielded a 6.53 cm RMSE. The crossings analyses in ArcGIS Pro were solely used as a QA/QC. The data were further cleaned in ArcGIS Pro to remove any loops, tight turns, or data that were off the planned lines, and point datasets was exported as the final data product (xyz text file). The beach xyz DGPS data were similarly merged into a single file, imported into ArcGIS Pro, and analyzed using the crossing program with a search radius of 3 m. For the beach data, any time survey tracks crossed, the crossing analysis yielded a 19.04 cm RMSE.

Datum Transformation: NOAA NGS's VDatum was used to transform the SBES and beach DGPS data points' horizontal and vertical datums (xyz). The data were transformed from WGS84 (G2139) UTM 18N to NAD83 (horizontal) State Plane Virginia (FIPS 4502) and NAVD88 (vertical) using GEOID18 with a reported vertical uncertainty of 0.119 feet (SBES) and 0.122 feet (DGPS). 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.