Coastal Bathymetry Data Collected in June 2018 from Fire Island, New York: Wilderness Breach and Shoreface

GPS Acquisition: A total of four Geographic Positioning System (GPS) base stations were established throughout the survey area, three of which were located on NPS established benchmarks, and one on a USGS-installed benchmark. Located in the western portion of the survey area, NPS benchmark BBDP75 or REST is established near the town of Robins Rest. NPS established benchmarks BBDX67 (HILL) and BBDX09 (DX09) are located near the western dockage at the Watch Hill NPS station. USGS-established benchmark FIVC, is a P/K nail located on the boardwalk leading to the Fire Island Visitor Center complex on the eastern side of wilderness breach. All base stations were occupied for 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 Number (FAN) 2018-322-FA, which encompass data from three separate survey platforms; The RV Shark (18CCT02), a 12-ft Yamaha PWC collected 325.25 line-km (156 lines),the RV Chum (18CCT03), an additional 12-ft Yamaha PWC collected 347.47 line-km (162 lines), and a seismic sled towed by the RV Sallenger (sled FAN 18CCT04) collected 288.44 line-km (98 lines). Boat motion was recorded at 50-millisecond (ms) intervals using a SBG Ellipse A #1 motion sensor aboard each PWC and the sled. HYPACK (version 18.1.8.0), 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 three SonTek Castaway Conductivity, Temperature, and Depth (CTD) instruments. The instruments were periodically cast overboard to observe changes in water column speed of sound (SOS). A total of 193 successful sound velocity casts were taken throughout the survey at an average depth of 8.14 meters, and on average produced a sound velocity of 1502.47 meters per second (m/s).

Ground Based Kinematic GPS Acquisition: Elevation data were collected from shallow flood shoals and channels, using two SECO GPS backpacks containing Ashtech Z-Xtreme receivers with Ashtech Marine antennas attached to a pole extending above the head of the surveyors (sub-FAN 18CCT05, BCK1 and BCK2). Positions were recorded at 10 Hz. The elevation of the antenna relative to the ground was the average measurement of the surveyor?s standing and stride height, measuring 1.868 m for Backpack 1 and 1.740 m for Backpack 2. The surveyors did not follow a pre-defined path but collected data over as much of the subaerial and shallow shoals as possible, during low tide.

Differentially Corrected Navigation Processing: The coordinate values of the GPS base stations are the time-weighted average of values obtained from NGS? OPUS. The base station coordinates were imported into GrafNav version 8.7 (Waypoint Product Group) and the kinematic GPS data from the survey vessel or backpack apparatus 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 0.1 s and exported in ASCII text format. Concurrent post-processed navigation data to single-beam data points replace the uncorrected rover positions, recorded during acquisition, in subsequent processing steps. The GPS data were processed and exported in the WGS84 G1150 (ITRF00) geodetic datum.

All data were processed using CARIS HIPS and SIPS (Hydrographic Information Processing System and Sonar Information Processing System) version 10.4.6. The raw HYPACK data files were imported into CARIS, the differentially corrected navigation files were imported using the generic data parser tool, 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, Quality Assurance (QA/QC) and Uncertainty Analysis: All single-beam data exported from CARIS, and elevation data exported from GrafNav were imported into Esri ArcMap version 10.6.0, where a shapefile of the individual sounding data points (x,y,z) was created and plotted in 0.25-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. Erroneous, ground-based horizontal and vertical positions, such as those that occurred when the surveyor took off the backpack and was transported between shoals, were removed. Next, a trackline shapefile was produced using X-tools Pro "Make Polylines from Points " function for each survey platform. Utilizing both the x,y,z (point) and trackline (polyline) shapefiles, a Python script evaluated elevation differences at the intersection of crossing tracklines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation with a search radius of 1 m. The RMS error for WVR1 (18CCT02), when crossing a trackline it previously surveyed was 14.6 cm and when WRV2 (18CCT03) crossed a trackline it previously surveyed, the RMS error was 14.2 cm. Data acquired by the sled (18CCT04) has an RMS error of 14.5 cm, and the GPS Backpacks have a 6 cm and 2.9 cm RMS error respectively. Please see the Error Analysis table provided in the supplemental Information section of this data release for other crossing error statistics. Once individual platform statistics were obtained, all data within the wilderness breach were merged and crossing analysis yielded a 11.27 cm RMS error. Additionally, all shoreface data were merged and reported an 18.8 cm RMS error. Since the bias between the platform elevations was on the order of the Odom instrument accuracy (1 cm +/- 0.7 percent depth), no adjustments were made. These merged files were exported from Esri ArcMap as an x,y,z text (.txt) file and made available in the download section of this data release, along with the populated trackline shapefiles.

Datum Transformation: NOAA's VDatum v.9.9 was used to transform single-beam and GPS elevation data points (x,y,z data) from their data acquisition datum (WGS84 ITRF00) to the North American Datum of 1983 (NAD83) reference frame and the North American Vertical Datum of 1988 (NAVD88) elevation using the National Geodetic Survey (NGS) geoid model of 2012A (GEOID12A). For conversion from the WGS84 ellipsoid to NAVD88, there is a total of 7.616 cm of uncertainty in the transformation (http://vdatum.noaa.gov/docs/est uncertainties.html). Resultant data files have been made available in the downloads section of this data release.

Create Digital Elevation Model: The transformed wilderness breach and shoreface x,y,z data described in the previous processing step were imported into ArcMap using the "Create Feature Class From XY Table" tool. The dataset was then used to create a triangulated irregular network (TIN) using the "Create TIN" tool. The TIN was subsequently converted into a raster file using the "TIN to Raster" tool with a cell size of 100 meters for the shoreface and 25 m for the wilderness breach. The rasters were then exported as an ASCII file using the "ASCII to Raster" tool. In order to ensure no data cells were populated without sufficient data, the Raster ASCII file was imported into Matlab (2015b) and any interpolated grid cells that were more than two cell sizes (50 m or 200 m) away from a data point were set to Not a Number (NaN). The raster data were then exported as an ArcGIS ASCII file from Matlab (2015b) and imported into ArcMap using the "ASCII to Raster" tool. The created grid was then exported from ArcMap as a 32-bit floating GeoTiff, utilizing the Natural Neighbor algorithm. A bounding box was generated for each DEM using Global Mapper version 19.0.2 ?export BBOX/COVERAGES tool. The created feature was then exported as a shapefile and included with the data downloads. When each DEM was compared to the associated x,y,z data, the shoreface DEM yielded a 0.383 m RMS error and the wilderness breach yielded a 0.244 RMS error.

Added keywords section with USGS persistent identifier as theme keyword.