Single-Beam Bathymetry Data 30-meter DEM Collected in 2015 from Grand Bay, Alabama/Mississippi

DGPS Navigation Processing: The Geographic Positioning System (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 north-north west 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). GPS instrumentation was duplicated on both survey vessels (rovers); however, an Ashtech Global Navigation Satellite Systems (GNSS) antenna was used instead of Choke-ring antennas. The base receivers and rover receivers recorded positions concurrently at all times throughout the survey. Rovers RV Shark and RV Chum recorded data every 0.1 s throughout the survey. The coordinate values for each of the GPS base stations (B166,189A) are the time-weighted average of values obtained from the National Geodetic Survey's (NGS) On-Line Positioning User Service (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 8.5 (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 at their respective time intervals (0.1-s for 15CCT04, 15CCT05), 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 16N.

Single Bream Processing: The raw HYPACK? data files were imported into CARIS HIPS and SIPS? (Hydrographic Information Processing System and Sonar Information Processing System) version 9.0.17. The corrected DGPS positions exported from GrafNav were imported into CARIS using the generic data parser tool. After parsing, the navigation data was scanned using the Navigation Editor allowing the user to view multiple types of plots including trackline orientation, timing, and course direction. This check verifies if the parsed data corresponds to the processed DGPS. Next, Speed of Sound Profile (SVP) casts were entered, and edited, using the SVP editor tool, and then applied as nearest in distance within time. All soundings are referenced to the ellipsoid during processing. This involved a step in CARIS to compute the GPS tide. The GPS tide represents the ellipsoidal surface. GPS tide and GPS height are then compared against each other to ensure correct computation by the program and applied GPS antenna height provided in the vessel file. All bathymetric data components including position, depth, GPS tide, and Speed of Sound (SOS), were then merged and geometrically corrected in CARIS to produce processed x,y,z point data. Once merged the dataset is reviewed for erroneous points using the Single Beam Editor. The points that are visually obvious are often related to cavitation in the water column obscuring the fathometer signal, tight turns at the marsh face affecting the tracking of the incoming GPS signal, and/or false readings due to general equipment issues. Data showing these issues are either discarded or adjusted to surrounding sounding depths. Also, data points in areas of extremely shallow water (0.30 m to 0.50 m) such as on shoals or within seagrass beds were reviewed against the surrounding data for overall consistency. Finally, a Bathymetry with Associated Statistical Error (BASE) surface was created. Using the Subset Editor, the BASE surface was used as a color coded guide to pinpoint crossings that are visually offset from one another. If an offset was identified, it was further examined and was reprocessed if necessary. The geometrically corrected point data are then exported as an x,y,z ASCII text file referenced to WGS84 (G1150), equivalent to ITRF00, UTM 16, and ellipsoid height, in meters. The combined single-beam bathymetry datasets (15CCT04 and 15CCT05) consists of 3,844,122 x,y,z data points with an ellipsoidal height range of -33.364 m to -29.531 m.

Quality Control and Quality Assurance (QA/QC) and Datum transformation: All Single-beam data found in the ASCII exported from CARIS were imported into Esri ArcMap version 10.2, 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. Then, the data were run through an in-house script created in Visual Basic (Crossing program Version 3.2). The script was created 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. Elevation values at line crossings should not differ by more than the combined instrument acquisition error (per manufacturer specified accuracies). 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 with both vessels collectively by use of a merged shape file with all point data (Grand_Bay_2015_SBB_Level_05_xxx_ITRF00). Once all data were found to be final, the single-beam bathymetric data were transformed horizontally and vertically from their data acquisition datum WGS84 (ITRF00) to the North American Datum of 1983 (NAD83) reference frame using the National Geodetic Survey (NGS) geoid model of 2012A (GEOID12A) as well as North American Datum of 1983 (NAD83) Mean Lower Low Water, using the transformation software VDatum version 3.2. The MLLW x,y,z data points were written into a ASCII format and considered to be the final values.

Gridding Bathymetric data and computing grid error: The single-beam soundings were imported into ESRI?s ArcMap version 10.2 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 created into a TIN using the "create TIN" and then the data were converted into a raster DEM by use of the "TIN to raster tool" using the natural neighbor function with a cell size of 30 m. Finally the interpolated DEM is clipped to the survey extent by use of the "extract by mask" tool using 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 -0.01 m maximum to -3.62 m minimum. 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 sounding (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. The overall RMS Error in meters is 0.07.

Added keywords section with USGS persistent identifier as theme keyword.