Single-Beam Bathymetry XYZ Data Collected in 2015 from Grand Bay, Alabama/Mississippi

GPS Acquisition: The Geographic Positioning Systems (GPS) base stations were occupied by USGS personnel for the purpose of this survey. Bench mark B166 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 (GBNERR), 189A was located northwest of the survey area at the boat launch location used for this survey. The base stations were equipped with Ashtech Proflex GPS Receivers recording 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via Thales choke-ring antennas, recording at 0.1 seconds (s), using WGS84 (G1150), equivalent to ITRF00, and ellipsoid height, in meters.

Single-beam Bathymetry Acquisition: The single-beam bathymetric data were collected under USGS FAN 2015-315-FA, which encompasses two sub-cruise identifiers specific to platform (15CCT04, 15CCT05). The RV Shark (15CCT04), a 12-foot (ft) Yamaha Personal Water Craft (PWC) collected 298.8 line-km (158 lines), and the RV Chum (15CCT05), an additional 12-ft Yamaha PWC collected 312.6 line-km (123 lines). A motion sensor device was not used for either vessel. Rather, to minimize motion errors, both RV Shark and RV Chum recorded GPS data at 0.1-seconds, utilized a short antenna height by use of a lever arm, and was outfitted with narrow (4-degree) transducer beam. HYPACK (version 13.0.09.17), 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 vessel 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, and fathometer were recorded in real-time aboard both 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 two 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 sound velocity casts were taken throughout the survey in water depths ranging from 0.135 to 3.95 m. The average sound velocity recorded was 1521.4 meters per second (m/s), and values ranged from 1515.6 to 1527.0 m/s.

Differentially Corrected Navigation Processing: The coordinate values of the GPS base stations (B166, 189A) are the time-weighted average of values obtained from 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 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 the respective time intervals of the roving GPS (0.1-s for 15CCT04, 15CCT05) and exported in ASCII text format to replace the uncorrected rover positions recorded during acquisition. The GPS data were processed and exported in the World Geodetic System of 1984 (WGS84 G1150) geodetic datum.

Single-beam Bathymetry Processing: All data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 9.0.17. The raw HYPACK (version 13.0.09.17) data files were imported into CARIS, the differentially corrected navigation files were imported using the generic data parser tool within CARIS, and any SVP profile casts were entered and edited using the SVP editor. The bathymetric data components (position, 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 (G1150), equivalent to ITRF00, and ellipsoid height, in meters.

Quality Control and Quality Assurance (QA/QC): All single-beam data found in the ASCII file exported from CARIS were imported into Esri ArcMap version 10.3.1, where a shapefile of the individual data points (x,y,z) was created and plotted in 1-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. A Python script was used for the purpose of evaluating elevation differences at the intersection of crossing tracklines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation. GPS cycle slips, stormy weather conditions, and rough sea surface states can contribute to poor data quality. If discrepancies that exceed the acceptable error threshold were found, then the line in error was either removed or statically adjusted. The script was run on all vessel point data first on a vessel by vessel basis and then run a final time for all data points from all vessels collectively. Once the dataset passed all QA/QC procedures and manual editing steps, the data were considered final and included in the download files of this data release.

Datum Transformation: The text file of the final XYZ soundings, was converted using version 3.6 of NOAA’s VDatum software conversion tool (reported vertical transformation error is 5.4 cm) from ITRF00 to the NAD83 reference frame and NAVD88 orthometric height using the NGS geoid model of 2012 (GEOID12A) and MLLW.

Added keywords section with USGS persistent identifier as theme keyword.