Coastal Single-beam Bathymetry Data Collected in August 2018 from the Chandeleur Islands, Louisiana

GPS Acquisition: One Geographic Positioning System (GPS) base station was established on USGS benchmark MRK3, located within the northern portion of the Chandeleur Islands. 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 2018-350-FA and encompasses data from a single survey platform, the RV Jabba Jaw, a 21 ft shallow draft center console catamaran (18BIM02), which surveyed a total of 143.726 line-km (66 lines). Boat motion was recorded at 50-millisecond (ms) intervals using a TSS 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. 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 the vessel 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 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 44 successful sound velocity casts were taken throughout the survey at an average depth of 4.47 m, and on average produced a sound velocity of 1537.12 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 vessels 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. 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 WGS84 (G1762) geodetic datum.

All data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 11.0.7. 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 profile 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 xyz data. TSS Motion Reference Unit (MRU) malfunction on Day 229 was corrected by applying a 90-degree offset to all effected data. Data associated with the correction are computed to have unreasonably high TPU values due to the applied offset. 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 xyz 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, where a shapefile of the individual sounding data points (xyz) 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 trackline shapefile was produced using X-tools Pro "Make Polylines from Points" function for each survey platform. Utilizing both the xyz (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 root mean square error (RMSe) when crossing a trackline it previously surveyed was 11.79 cm. Since the bias between elevations was on the order of the Odom CV100 instrument accuracy (1 cm +/- 0.7 percent depth), no adjustments were made. These merged files were exported from Esri ArcMap as an xyz text (.txt) file and made available in the download section of this data release, along with the populated trackline shapefiles.

Datum Transformation: NOAA's VDatum v.3.9 was used to transform single-beam and GPS elevation data points (xyz) from their data acquisition datum (WGS84 G1762) to the NAD83 reference frame with NAVD88 elevations using the NGS geoid model of 2009 (GEOID09). For conversion from the WGS84 ellipsoid to NAVD88, there is a total of 7.616 cm of uncertainty in the transformation (http://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.