Coastal Bathymetry Data Collected in 2016 from the Chandeleur Islands, Louisiana–Single Beam Bathymetry Soundings (XYZ)

GPS Acquisition: A USGS-installed benchmark, CTMP, was occupied on the northern Chandeleur Islands. The base was equipped with an Ashtech ProFlex GPS receiver, which recorded the 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via the Thales choke-ring antenna at a rate of 2 times per second. GPS instrumentation on the rover vessel recorded concurrently at 2 times per second. The GPS navigation data were acquired and processed in the current realization of WGS84 (G1150).

Single Beam Bathymetry Acquisition: Depth soundings were acquired aboard the vessel at 100-milliseconds (ms) using an Odom CV100 echo sounder system with a 200 kilohertz (kHz) transducer. Boat motion was recorded on the R/V Jabba Jaw (16BIM02) using a TSS motion sensor. All sensor data were combined and saved into a single raw data file (.raw) in HYPACK version 2014, with each device string referenced by a device identification code and time stamped to Coordinated Universal Time (UTC). Sound velocity measurements were collected using a Sontek Castaway CTD unit to record changes in water column speed of sound (SOS).

Differentially corrected navigation processing: The coordinate values of the GPS base station, CTMP, are the time-weighted average of values obtained from OPUS. These coordinates were entered into the post-processing software, GrafNav version 8.6. The kinematic GPS data from the survey vessel was then processed to the static GPS data from the CTMP base station. Steps were taken to ensure that the trajectories between the base and rover were clean and resulted in fixed positions. GPS data quality was monitored and maintained by analyzing the graphs, trajectory maps, and processing logs that GrafNav produces for each GPS session. If poor GPS data points were identified during the quality assurance/quality control (QA/QC) process, some common tools used to improve the solution were utilized. These options included, but were not limited to, omitting a satellite flagged as poor in health, excluding time-segments with cycle slips, or adjusting the satellite elevation mask angle. The final, differentially-corrected, precise DGPS positions were computed for the roving vessel’s recording rate, and then exported in ASCII text format. The exported ASCII file subsequently replaced the uncorrected real-time rover positions (recorded at the time of acquisition in ITRF00) during post-processing. The GPS data were processed and exported in WGS84 (G1150), which is equivalent to ITRF00, and ellipsoid height in meters.

Single beam bathymetry processing: All data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 9.1. The raw HYPACK 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 imported and applied. The bathymetric data components (position, motion, depth, and SOS) were then merged and geometrically corrected 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), which is equivalent to ITRF00, and ellipsoid height in meters. The exported ASCII files from the vessel were later combined with the IFB data file for further QA/QC.

Single beam bathymetry error analysis: The ASCII files described in the previous step were imported into ArcMap version 10.3 and reviewed for outliers. An Esri ArcMap add-in program script was written locally, by USGS staff, to evaluate the elevation differences at the intersection of crossing lines. If discrepancies were found at a crossing, the source of the discrepancy was identified and, if needed, the line or segment of data points in error was statically adjusted or removed. This process was reiterated again to produce a final assessment of the crossing range for the SBB dataset.

VDatum version 3.6 transformation: The SBB data were transformed two times, using VDatum. The vertical transformation errors reported by VDatum are the following: 1) Transformation from ITRF00 to NAD83, NAVD88 with respect to GEOID09 is 0.076158 m or 7.6158 cm and 2) Transformation from ITRF00 to ITRF08 is 0.000 m or 0.00 cm.

The 16BIM02 trackline files were created in Esri ArcGIS version 10.3.1 from the x,y,z point files by converting the x,y,z data into shapefiles. Using the XTools Pro extension, version 11.1 feature conversion tool, "make polylines from points" the x,y,z data point shapefile was converted into a polyline shapefile. This was repeated for both datum transformations, ITRF08 and NAD83 for the data release.

Added keywords section with USGS persistent identifier as theme keyword.