Coastal Single-beam Bathymetry Data Collected in August 2019 from Cedar Island, Virginia

GPS Acquisition: One GPS base station was established on NGS benchmark AJ4587 (https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AJ4587), located within Seaside Park at the corner of Main Street and Atlantic Avenue, Wachapreague, VA. The base station was continually occupied and equipped with an Ashtech Proflex 800 GPS receiver recording 12-channel 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 USGS FAN 2019-332-FA and encompasses data from a single survey platform, a Chirp seismic sled, towed by the R/V Sallenger, a 26 ft fiberglass research vessel. A total of 299.075 line-km (62 lines) were surveyed. Boat motion was recorded at 50-millisecond (ms) intervals using an SBG motion sensor. 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 aboard the R/V Sallenger. Depth soundings were recorded at 50-ms intervals using an Odom Echotrac CV100 echo sounder with a 200-kilohertz (kHz) 4-degree transducer. Data from the GPS receiver, motion sensor, and echo sounder 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 a single SonTek Castaway conductivity, temperature, and depth (CTD) instrument. The instrument was periodically cast overboard to observe changes in water column speed of sound (SOS). A total of 26 successful sound velocity casts were taken throughout the survey at an average depth of 3.99 m, and on average produced a sound velocity of 1519.08 meters per second (m/s).

Differentially Corrected Navigation Processing: The coordinate value of the GPS base station is the time weighted average (TWA) of all current survey occupations. The base station coordinates were imported into GrafNav version 8.7 (Waypoint Product Group) and the kinematic GPS data from the survey platform were post-processed to the concurrent GPS session data at the base station. During processing, steps were taken to ensure that the trajectories between the base and the rover were clean, resulting 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. The GPS data were processed and exported in the WGS84 (G1762) geodetic datum.

All bathymetry data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 11.2.3. The raw HYPACK data files were imported into CARIS, the differentially corrected navigation files were imported using the generic data parser (GDP) tool, and any SVP profile casts were entered and edited using the SVP editor. Lines from survey day 222 (8/10/2019) utilized real-time navigation due to GPS errors associated with a system failure. To utilize real-time navigation, the GPS heights were extracted from the raw Hypack (.RAW) files utilizing the CARIS command Line GUI (Version 2.0.4), and then written into an ASCII text file, which could be read by the GDP. Once GPS heights were applied using the GDP, a smoothing (moving average) filter of 60 seconds and a threshold of 1 was applied to data. The bathymetric data components (position, motion, depth, and SOS) for all data 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): All single-beam data exported from CARIS were imported into Esri ArcMap version 10.6.0, where a shapefile of the individual sounding data points (x,y,z) was created and plotted in 0.25-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. Next, a track line shapefile was produced using X-tools Pro "Make Polylines from Points" function. Utilizing both the x,y,z (point) and track-line (polyline) shapefiles, an in house Python script evaluated elevation differences at the intersection of crossing track lines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation with a search radius of 1 m. The root mean square error (RMSe) when crossing a track line it previously surveyed was 23.5 cm. Lines from survey day 222 (8/10/2019), which utilized real-time navigation, had crossing offsets that exceeded the published Odom CV100 instrument accuracy (1 cm +/- 0.7 percent depth) and therefore were statically (vertical) adjusted by the average crossing offset. The following vertical adjustments were made, Line 0048_1731: -0.3826 m, 0049_1709_0001: -0.4578 m, 0050_1648: -0.4182 m, 0004_1930: +0.2767 m and Line 0001_1425: -0.2764 m. Once these adjustments were made, the crossing program was run for a second time, which produced RMSe results of 19.86 cm crossing error, however 91% of all crossings are under 10 cm. The final adjusted file was exported from Esri ArcMap as an x,y,z text (.txt) file and made available in the download section of this data release, along with the populated track line shapefiles.

Datum Transformation: NOAA's VDatum v.3.9 was used to transform single-beam data points (x,y,z) from their data acquisition datum (WGS84 [G1762]) to the NAD83 reference frame with NAVD88 elevations using the NGS geoid model 2012B (GEOID12B). For conversion from the WGS84 ellipsoid height to NAVD88 orthometric height, there is a total of 7.616 cm of uncertainty in the transformation (https://vdatum.noaa.gov/docs/est_uncertainties.html). Resultant data files have been made available in the downloads section of this data release.

Added keywords section with USGS persistent identifier as theme keyword.