Nearshore Single-Beam Bathymetry XYZ Data Collected in 2017 from the Chenier Plain, Louisiana

GPS Acquisition: A total of 8 Geographic Positioning Systems (GPS) base stations were established throughout the survey area, 5 of which were located on NGS established marks, 1 on a USGS installed mark, and 2 on other marks within the Rockefeller National Wildlife Refuge. Located in the eastern portion of the survey area, NGS mark DN4165 is established at the mouth of the freshwater lock to the west of Marsh Island. USGS established mark MIGP, installed in 2016, is located on the western shore of Marsh Island. Near the middle of the survey area, RFS1 (markings Fish Lab 1), and ME18 were located on the grounds of the Rockefeller National Wildlife Refuge. In the western portion of the survey area, NGS mark DJ9378 is established near the northern grass edge of the boat ramp at the southern end of Calcasieu Pass, this mark was found damaged with a bent rod tip. Because of the damage, coordinates derived from survey occupations were used in final processing. NGS mark DH3817 is located off Highway 27 in Cameron city limits and mark AV0360 is located in Holly Beach on the grounds of the torn down church, near the stop sign. The most western NGS mark utilized is DN4168 which is located off highway 82 past the high school near the Sabine Pass Bridge. All base stations were occupied 24 hours and 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 a rate of 0.1 seconds (s).

Single-Beam Bathymetry Acquisition: The single-beam bathymetric data were collected under the USGS Field Activity Numbers (FAN) 2017-323-FA and 2017-324-FA, which encompass data from four separate survey platforms; The RV Sallenger (17BIM01, 17BIM05), a 26-foot (ft) Parker, collected 1,202.80 line-km (378 Lines), RV Jabba Jaw (17BIM02, 17BIM07), a shallow draft 22-ft Twin Vee collected 906.87 line-km (338 lines), the RV Shark (17BIM03, 17BIM07), a 12-ft Yamaha Personal Water Craft (PWC) collected 568.64 line-km (174 lines), and the RV Chum (17BIM04, 17BIM08), an additional 12-ft Yamaha PWC collected 720.38 line-km (251 lines). Boat motion was recorded at 50-millisecond (ms) intervals using a Teledyne TSS Dynamic Motion Sensor (TSS DMS-05) on the R/V Sallenger, R/V Jabba Jaw, and by use of a SBG Ellipse A #1 motion sensor aboard each PWC. HYPACK (version 16.1.8.0), 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 on all vessels. Data from the GPS receiver, motion sensor, and fathometer were recorded in real-time aboard all 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 four SonTek Castaway Conductivity, Temperature, and Depth (CTD) instruments as well as one Valport mini sound velocity profiler (SVP). The instruments were periodically cast overboard to observe changes in water column speed of sound (SOS). A total of 392 successful sound velocity casts were taken throughout the survey at an average depth of 5.06 meters, and on average produced a sound velocity of 1519.68 meters per second (m/s).

Differentially Corrected Navigation Processing: The coordinate values of the GPS base stations are the time-weighted average of values obtained from the NGS?s 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 0.1s and exported in ASCII text format to replace the uncorrected rover positions recorded during acquisition. The GPS data were processed and exported in the 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 10.2.1. 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 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 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, ellipsoid height in meters.

Quality Control and Quality Assurance (QA/QC): All single-beam data 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 0.5-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. Next, a Python script 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. GPS cycle slips, stormy weather conditions, and rough sea surface states can contribute to poor data quality. In all cases where discrepancies in the data, or high (>0.30 m) crossing values were found, there was sufficient data coverage to delete the problem data points and/or lines. 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 within a merged file. A total of 4,406 crossing values were observed throughout the survey area, 95% of which are less than 0.30m. Once the dataset passed all QA/QC procedures and manual editing steps, the data were considered final and included in the download section of this data release.

Datum Transformation: The text file, Chenier_Plain_2017_SBB_Level_03B_xxx_ITRF00.txt, was converted using NOAA?s VDatum software conversion tool version 3.6 (reported vertical transformation error is 7.6158 cm) from ITRF00 to the NAD83 reference frame and NAVD88 orthometric height using the National Geodetic Survey (NGS) geoid model of 2012A (GEOID12A).

Added keywords section with USGS persistent identifier as theme keyword.