Fire Island Shoreface Bathymetric Data collected with Personal Watercraft and Backpack along Fire Island, New York (2014) as a GeoTIFF

GPS Acquisition: Horizontal and vertical positioning of each vessel and backpack was were collected determined using a base-rover configuration. Data were recorded at 10 Hertz (Hz) using Ashtech ProFlex™ 500 Global Navigation Satellite System (GNSS) receivers with Thales choke ring antennas. Three stationary base stations (REST, VC, and U374) were occupied during the surveys. The stationary base at published NGS benchmark U374 (Permanent Identification number (PID#) KU0206) was equipped with an Inmarsat Broadband Global Area Network (BGAN) satellite uplink system for remote monitoring of the base station. U374 consisted of an Ashtech Proflex 500 GNSS receiver and an Ashtech choke ring antenna with a vertical offset of 1.24 meters (m). GPS data acquired by the PWCs, backpack, wheel-mount, and the REST and VC base stations were downloaded at the end of each survey day. A small segment of the U374 data was downloaded via the BGAN network nightly to ensure the system was operating properly. Reference station coordinates were verified with Continuously Operating Reference Stations (CORS) stations using OPUS, (http://www.ngs.noaa.gov/OPUS/). OPUS computed reference positions had a vertical error of 0.007 m and horizontal errors of 0.8 cm and 0.6 cm for East-West and North-South, respectively.

Single-Beam Sounding Acquisition: The single-beam bathymetric data were collected on two Yamaha (2010 and 2013) VX Deluxe personal watercraft (PWC). HYPACK version 2013 was used for positioning and navigation during the survey. Depth soundings were recorded at 10 Hz using an Odom Ecotrac CV-100 Digital Hydrographic Echo Sounder system with 200 kiloHertz (kHz) transducers with 4-degree (vessel 1) and 9-degree (vessel 2) transducers. Soundings were merged into a raw data file (.raw) and a sounding file (.bin) in HYPACK. Each file was named according to transect number and coordinated universal time (UTC). Water column sound velocity measurements were collected periodically throughout the survey, using a SonTek CastAway conductivity, temperature, and depth (CTD) sensor. Data were processed using SonTek CastAway CTD software version 1.5.

Backpack GPS Acquisition: Elevation data were collected from shallow flood shoals and channels, using a SECO GPS backpack containing an Ashtech Z-Xtreme receiver with Ashtech Marine antennas attached to a pole extending above the head of the surveyor. Positions were recorded at 10 hertz (Hz). The elevation of the antenna relative to the ground was measured for the surveyor in a walking stride position (2.112 m). 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.

Wheel-Mounted GPS Acquisition: A wheel-mounted GPS system containing an Ashtech Z-Xtreme receiver with Ashtech Marine antennas was used to record elevations and positions at 10 Hz. The elevation of the antenna relative to the ground was fixed at 2 meters. The system collected data along the approximate mean high water (MHW) line, along pre-defined transects, and additional data on the subaerial beach.

Single-Beam Differentially Corrected Navigation Processing: Positions and elevations associated with each sounding were post-processed using differential corrections derived from the base/rover setup. Applying the reference station coordinates, GPS data acquired from the rover were processed to the concurrent GPS session data at the base station- using GrafNav version 8.5 software (Waypoint Product Group). The horizontal and vertical coordinates were recorded in the World Geodetic System of 1984 (WGS84) reference frame and exported as an ASCII file for each vessel and each survey day.

Ground-Based Differentially Corrected Navigation Processing: GPS data points were post-processed using a differential correction derived from the base/rover setup. The base station coordinates were imported into GrafNav version 8.5 (Waypoint Product Group) and the GPS data from the backpack and wheel-mounted systems were processed to the concurrent GPS session data at the base stations. The horizontal and vertical coordinates of the ground-based data were saved in NAD83 and NAVD88 and exported as ASCII files.

Ground-Based GPS Processing: Using ArcGIS, position and elevation of the ground based data were analyzed for instances when the surveyor was either sitting, removing the backpack, being transported between shoals on a personal watercraft, or tilted the wheel horizontally. Once all extraneous data points were removed, the remaining data were saved as an ASCII file.

Single-Beam Processing: Soundings were merged with processed DGPS data and sound velocity profiles using Matlab to visually analyze single-beam soundings and correct for errors such as elevation outliers and dropouts associated with wave breaking in the surf zone. When this is suspected, a corrected seafloor elevation was manually digitized by analyzing the complete waveform signal recorded by the Odom within the .bin data file. The soundings were then corrected for the speed of sound associated with the mean water temperature and salinity. A moving average filter was then applied to the soundings in order to reduce instrument noise and noise associated with the pitch and roll of the PWC. The soundings were referenced to the height of the GPS antenna and subsequently to the WGS84 ellipsoid.

Single-Beam Datum transformation: NOAA’s VDatum v3.3 was used to transform single beam data points (x, y, and z data) from their acquisition datum (WGS84) 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 5.4 cm of uncertainty in the transformation (http://vdatum.noaa.gov/docs/est_uncertainties.html).

Single-Beam Error Analysis: The accuracy of the single-beam soundings was evaluated by identifying locations where survey track lines either crossed or were within a horizontal distance of 0.25 m of each other. Any track line associated with a crossing that had an elevation differing by greater than 0.6 m, compared to crossing lines, was removed. Evaluation of the remaining track line crossings indicated there was an overall mean difference of 3.8 cm (based on 210 crossings) and (root mean square) RMS error of 16.5 cm (based on 696 crossings). Applying the square root of the sum of the datum conversion uncertainty (5.4 cm) and the sounding uncertainty (16.5 cm) results in a combined vertical error of 17.4 cm. Horizontal uncertainty is assumed to be at most half of the vertical uncertainty (8.7 cm).

Ground-Based Uncertainty Calculation: Backpack and wheel GPS elevation errors were calculated by computing the vertical differences at crossings occurring at least 1 minute apart. Using Matlab, the calculated RMS error was 12.5 cm. Elevation differences between the ground-based data points and single-beam data points indicate the backpack elevations were 5 cm higher than elevations recorded using PWCs. Given the high degree of uncertainty arising from the backpack surveyor striding over a subaqueous shoal surface and the tilting of the wheel-mount, the ground-based data were adjusted to the PWC elevation at the crossings. The adjusted positions, elevation, and date of sampling were saved as an ASCII file.

Export Transects: Using Matlab, partial lines (the result of restarting the line in the middle of a transect) were subsequently merged with similar segments to create one seamless line. When repeats were present, only a single line was retained. Vessel 1 elevations were adjusted to those of vessel 2 for consistency (3.8 cm). The data were then combined into a single ASCII file consisting of position, elevation, line number, vessel number, and time of sampling.

Extract Single-Beam Shoreface XYZ: The adjusted single-beam data points were imported into ArcGIS using the ”Create Feature Class from XY Table” tool in ArcCatalog. A polygon was then created surrounding data points within the Fire Island shoreface in ArcGIS. The polygon vertices were converted to points using the “Feature Vertices to Points” tool, “Add XY Coordinates” tool, and exported as an ASCII file using the “Export Feature Attribute to ASCII” tool. This polygon ASCII file was subsequently imported into Matlab. PWC data points within or on this polygon were then extracted and saved as an ASCII file.

Extract Ground-Based Shoreface XYZ: The adjusted ground-based data points were imported into ArcGIS using the “create feature class from xy yable” tool in ArcCatalog. A polygon was then created surrounding data points near the shoreline. The polygon vertices were converted to points using the “feature vertices to points” tool, “add xy coordinates” tool, and exported as an ASCII file using the “export feature attribute to ASCII” tool. This polygon ASCII file was subsequently imported into Matlab. Ground-based data points within or on this polygon were then extracted and saved as an ASCII file.

Create Raster: The Fire Island shoreface single-beam and ground-based x, y, and z data points were imported into ArcGIS using the “Create Feature Class From XY Table” tool in ArcCatalog. The dataset was then subsampled using the “Subset Features” and 10% of each data was removed to provide an estimate of the bathymetry uncertainty. The remaining 90% of each dataset was 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 50 meters. The Raster was exported as an ASCII file using the “ASCII to Raster” tool.

Remove Extrapolated Cells: The Raster ASCII file was imported into Matlab and any interpolated grid cells more than 2 cell sizes (100 m) away from a Fire Island shoreface data point was set to "not a number" (NaN) in Matlab. The raster data were then exported as an ArcGIS ASCII file from Matlab and imported using the "ASCII to Raster" tool.

Evaluate Raster Uncertainty: The raster was sampled at the positions of the 10% removed data using the "Extract Values to Points" and a .csv file was saved from each data table for the single-beam and ground-based data points. Using Matlab, the sampled data were compared to the subset data and the root mean square differences were calculated. The vertical RMS error was found to be 15.3 cm.

Convert Raster Format: In ArcCatalog, a raster dataset was created using the “Create Raster Dataset” tool with a .tif image format, 64 bit pixel type. The Fire Island shoreface raster was then imported into this dataset using the “Mosaic” tool.

Keywords section of metadata optimized by correcting variations of theme keyword thesauri and updating/adding keywords.

Added keywords section with USGS persistent identifier as theme keyword.