Chenier_Plain_2017_SBB_200m_DEM_metadata: Nearshore Single-Beam Bathymetry XYZ Data Collected in 2017 from the Chenier Plain, Louisiana

DGPS Navigation Processing: 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 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). The coordinate values for each of the GPS base stations are the time-weighted average of values obtained from the NGS?s OPUS. All base station sessions of recorded data are decimated to 30-seconds (s), and then submitted to OPUS via the online service. All solutions are then returned to the user and entered into a spreadsheet where time-weighted ellipsoid values are calculated for each station, for the entire occupation. Any individual ellipsoid value that falls outside three standard deviations for the entire occupation was excluded and the final coordinate values were then determined. The final 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 data from the base stations. During processing, steps were taken to ensure that the trajectories between the base and rover were clean and resulted 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 with cycle slips were excluded, and the satellite elevation mask angle was adjusted to improve the position solutions, when necessary. The final, differentially corrected, precise DGPS positions were computed and then exported in ASCII text format. The file was then used to replace the uncorrected real-time rover positions recorded during acquisition. The GPS data were processed and exported in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum UTM zone 15N.

Single-Beam 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 were additionally reviewed. The geometrically corrected point data were then exported as an x,y,z ASCII text file referenced to WGS84 (G1150), equivalent to ITRF00, Universal Transverse Mercator (UTM) 15, and ellipsoid height in meters. The combined single-beam bathymetry dataset consists of 14,345,969 x,y,z data points with an ellipsoidal height range of -56.83 m to -26.62 m.

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).

Gridding bathymetric data and computing grid error: The single-beam soundings were imported into Esri's ArcMap version 10.3.1 and gridded using Spatial Analyst tools "create TIN," "TIN to raster," and "extract by raster mask." First a bounding polygon representing the extent of survey tracklines was created and converted into a raster mask using the ArcGIS "polygon to raster" tool. Next, the final point data were constructed into a triangulated irregular network (TIN) using the "create TIN" tool and then the data were converted into a raster DEM by utilizing the "TIN to raster? tool and natural neighbor function with a cell size of 200 m. Finally, the interpolated DEM is clipped to the survey extent by use of the "extract by mask" tool and applying the raster mask created first in this process. The final product is a DEM of the entire survey extent. The grid range values were from -26.66 m to -68.94 m for the ITRF00 ellipsoid DEM and 0.00 m to -30.13 m for the NAVD88 GEOID12A DEM. In order to evaluate how well the final DEM represents the final sounding data both spatially and quantitatively, a comparison of the DEM versus the x,y,z point data was plotted in ArcGIS by use of the "Extract values by points" spatial analyst tool. This tool extracts the value represented by the underlying grid and compares it to that of the overlying point data. By use of the generated shape file and associated attribute table, the root mean square (RMS) error, quantified as the difference between the measured depth and the grid depth values, was calculated. Almost 97% of the point data are represented by the DEM and has an overall RMS of 0.51 m. It is known that gridding algorithms, by nature, do not account well for steep slopes or channel coverage, and when present, usually unnecessarily increase the RMS error value for the entire dataset. To better represent the actual RMS error on the continental shelf, known dredged channels were removed from the. The extraction process and RMS calculation were then repeated resulting in 90% point data utilization with an overall RMS error of 0.19 m.

Added keywords section with USGS persistent identifier as theme keyword.