Digital Elevation Model from Single Beam Bathymetry XYZ Data Collected in June 2015 from the Chandeleur Islands, Louisiana

DGPS Navigation Processing: Geographic Positioning Systems (GPS) base stations were re-occupied by USGS personnel for the purpose of this survey. The names of the benchmarks used were: MRK3, located to the north of the survey area on a bank near the Pelican fishing camp, SM01 located in the middle of the survey area on the landward shore of Monkey Bayou, and GRIG located in the southern portion of the survey area atop a retired oil drilling platform (http://pubs.usgs.gov/ds/1039/html/ds1039_ overview.html). 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). GPS apparatus was duplicated on all survey platforms with the exception of the Personal Water Craft (PWC) (15BIM03) in which a smaller, Ashtech Global Navigation Satellite Systems (GNSS) antenna was used. The base receivers and rover receivers recorded positions concurrently. R/V Chum recorded at 0.1 s throughout the survey while the R/V Jabba Jaw and the R/V Sallenger (JD 172-175) recoded at 0.2 s, R/V Sallenger recorded at 1.0 second the first 4 days of the survey (JD168-171). The coordinate values for each of the GPS base stations (MRK3, SM01, GRIG) 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 for efficient processing, 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. In all cases the closest base station to the roving vessel was utilized to help keep vertical error to a minimum. 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, 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 were scanned using the Navigation Editor, which allows 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 (SOS) 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 (WGS84, ITRF00) 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; position, motion, depth, GPS tide, and Speed of Sound (SOS), were then merged and geometrically corrected in CARIS to produce processed x,y,z data. Once merged the dataset is reviewed for erroneous points using the Single Beam Editor. The points that are visually obvious outliers are often related to cavitation in the water column obscuring the fathometer signal, tight turns in the surf zone affecting the tracking of the incoming GPS signal, and/or false readings due to general equipment issues. Data showing these 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 shoals and seagrass beds are reviewed against the surrounding data for overall consistency. Finally, a Bathymetry with Associated Statistical Error (BASE) surface is created. Using the Subset Editor, the BASE surface is used as a color coded guide to pinpoint crossings that are visually offset from one another. If an offset is identified, it is further examined and is 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 single-beam bathymetry datasets combined (15BIM01, 15BIM02, and 15BIM03) consists of 8,224,799 point elevations with an ellipsoidal elevation range of -43.927 to -25.966 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 1-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. A Python script was used 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. 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 by the average of the crossings within the line. 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. For this dataset, the collective run, using a merged shapefile of all vessel point data, is the most important as the survey structure did not allow for sufficient vessel crossing, rather vessels crossing other vessels. Once the dataset passed all QA/QC procedures and manual editing steps, the data were considered final. The final x,y,z (WGS84, ITRF00) Ellipsoid point data was exported into a ASCII text file by use of the ArcGIS extension "XTools Pro" Export table to file function and made available for download via the survey product page of the accompanying Data Series.

Transformation, Gridding and RMS Calculation: The final sounding data that contribute to the Digital Elevation Model (DEM) were transformed horizontally and vertically into North American Datum 1983 (NAD83) (CORS96), UTM Zone 16N and North American Vertical Datum 1988 (NAVD88) orthometric height using the GEOID12A geoid model using the National Oceanic and Atmospheric Association (NOAA) VDatum version 3.2 transformation software (reported vertical transformation error is 5.4 cm). The resulting orthometric heights (elevations) for all survey vessels ranged from 0.01 m to -15.40 m. The NAVD 88 point elevations were imported into ESRI’s ArcMap version 10.2, merged using the data management "merge" tool to create shape file (2015_317_FA_SBB_Level_03_1039_NAD83_NAVD88_GD12A.shp) and gridded using the 3D Analyst and Spatial Analyst toolsets. A triangulated irregular network (TIN) was created from the shapefile containing the final point data using the "create tin" tool, and the data were then converted into a digital elevation model (DEM) using the "tin to raster” tool and natural neighbors algorithm with a cell size of 200 m. A bounding polygon representing the extent of survey tracklines was created and converted into a raster mask using the ArcGIS "polygon to raster" tool, and the interpolated DEM was clipped to this survey extent using the "extract by mask" tool. The final product is a DEM of the entire survey extent; grid values range from -0.00 m maximum to -15.40 m minimum. In order to evaluate how well the DEM represents the final sounding data both spatially and quantitatively, a comparison of the DEM cell value versus the final point elevations were plotted in ArcGIS using 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. The root mean square (RMS) error, quantified as the difference between the measured depth and the grid depth values, was calculated for the entire survey using the generic RMS equation. Observed point elevations were subtracted from the values extracted from the grid, and the squared sum was divided by the number of samples represented by the grid. The RMS for the entire survey equals 0.23.

Added keywords section with USGS persistent identifier as theme keyword.