Wilderness Breach Bathymetry Data Collected in May 2015 From Fire Island, New York: 50-Meter Digital Elevation Model

Process Description: GPS Acquisition: GPS base stations were erected at benchmarks REST (near the town of Robins Rest) and U374 (NGS benchmark Permanent Identification number (PID#) KU0206) located on Fire Island. The base stations were equipped with Ashtech ProFlex 500 Global Navigation Satellite System (GNSS) receivers. The survey personal watercraft (PWC) (rovers) were equipped with ProFlex 500 GNSS receivers. The base and rover receivers recorded their positions concurrently at 10 Hertz (Hz) throughout the survey. Reference-station coordinates were verified with Continuously Operating Reference Station (CORS) sites using the Online Positioning User Service (OPUS), available at http://www.ngs.noaa.gov/OPUS/. U374 used reference stations ZNY1, NYRH, CTDA;, REST used stations NYCI, NYRH, and MOR6, and VC base used MOR6, CTGU, AND NYRH. OPUS-computed reference stations had horizontal errors of 0.4 cm for REST, 0.5 cm for U374, and 0.3 cm for VC. Vertical errors were 0.2 cm for REST, 0.3 cm for U374, and 1.5 cm for VC.

Single-Beam Sounding Acquisition: Nearshore bathymetry was measured using single-beam sonar systems and GPS receivers mounted on two Yamaha (2010 and 2013) VX Deluxe personal watercraft (PWC). Each PWC was fitted with a single-beam transducer mounted off the starboard stern below the waterline off the starboard stern and placed 1.23 m and 1.30 m beneath the GPS antenna, for PWC1 and PWC2, respectively. HYPACK version 2013 was used for positioning and navigation during the survey. Depth soundings were recorded at 10 Hertz (Hz) using an Odom Ecotrac CV100 Digital Hydrographic Echo Sounder system with 200 kHz transducers with 4-degree 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) meter. Data were processed using SonTek CastAway CTD software version 1.5.

Ground-Based Mapping: Elevation data were collected using two Ashtech Z-Xtreme GPS receivers mounted in SECO backpacks with Ashtech Marine antennas attached to a pole extending above the head of the surveyor. The elevation of the antennas relative to the ground was measured for each surveyor in a walking stride position (varying between 2.07 and 2.10 m). The surveyors did not follow a pre-defined path but collected data over as much of the subaerial and shallow shoals and beach as possible during low tide.

Ground-Based Differentially Corrected Navigation Processing: Locations associated with backpack position were postprocessed 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 surveys were processed to the concurrent GPS session data at the base stations. The horizontal and vertical coordinates of the backpack data points were saved in NAD83 and NAVD88 and exported as ASCII files.

Ground-Based Data Processing: Using ArcGIS, 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. Once all extraneous data points were removed, the remaining data were saved as an ASCII file.

Single-Beam Differentially Corrected Navigation Processing: Horizontal positions and vertical elevations associated with each single-beam sounding were postprocessed using differential corrections derived from the base/rover setup. Two GPS reference stations were used for the survey and were located at benchmarks U374 and REST. 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 personal watercraft and each survey day.

Single-Beam Data Processing: Single-beam soundings were merged with differentially processed GPS data and sound velocity profiles using Matlab (2015b). Each transect was visually inspected for elevation outliers and dropouts associated with wave-breaking in the surf zone and were corrected manually. Usually, the highest intensity return is generated by the surface of the seafloor. Breaking waves in the surf zone can create air bubbles in the water column and cause an erroneous peak in waveform intensity. Errors are then produced in the interpreted seafloor reflection. When this circumstance was suspected, a corrected seafloor elevation was manually digitized by analyzing the .bin data file, which contains the complete waveform trace recorded by the Odom. The soundings were then corrected for the speed of sound (table 1) associated with the mean water temperature and salinity. A moving-average filter was then applied to the soundings to reduce instrument noise and the depth variations associated with the pitch and roll of the PWC. The depth soundings (from the transducer to the seafloor) were then adjusted to the depth reading from 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,z data) from their data 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 tracklines crossed and soundings from each line were within a horizontal distance of 0.25 m of each other. Evaluation of the trackline crossings indicated there was a root mean square (RMS) vertical uncertainty of 18.3 cm with a 1-cm bias between the two PWCs. Large elevation differences often appear near channels where elevations vary rapidly over short distances. The RMS error for PWC2, when crossing a trackline it previously surveyed, was 13.0 cm. When PWC1 crossed a trackline it previously surveyed, the RMS error was 16.8 cm. Since the bias between the PWC elevations was on the order of the Odom instrument accuracy (1cm +/- 0.7 percent depth), no adjustments were made. Applying the square root of the sum of the datum conversion uncertainty and the sounding uncertainty resulted in a combined vertical error of 17.7 cm. Horizontal uncertainty was assumed to be half of the vertical uncertainty (8.9 cm), at most.

Merging Transects: Using Matlab (2015b), partial lines (the result of restarting the line in the middle of a transect) were subsequently merged with similar segments to create single, seamless lines. When repetitions were present, only a single line was retained. The data were then combined into a single ASCII file consisting of position, elevation, line number, vessel number, and time of sampling.

Ground-Based Error Analysis: Using Matlab (2015b), ground-based GPS elevation errors were calculated by segmenting temporally sequential elevations into 10-minute segments and identifying crossings between segments where an elevation was within at least 0.25 m. The calculated RMS of those intersections is 8.5 cm. Elevation differences between the ground-based and single-beam data points indicated the ground-based elevations were 1.8 cm higher than elevations recorded using PWCs. Given the high degree of uncertainty arising from variations in the stride of the backpack surveyor over a subaqueous surface, the data were adjusted to the single-beam elevation at the crossings. The adjusted positions, elevation, and time of sampling were saved as an ASCII file.

Extract Wilderness Breach XYZ: Single-beam x,y,z data were imported into Esri’s ArcGIS using the “create feature class from xy table” tool, in ArcCatalog. Using ArcMap, a polygon was then created surrounding the data points within the shoreface of Fire Island. 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 ASCII file was subsequently imported into Matlab (2015b). Any single-beam data within or on this polygon were then extracted and saved as an ASCII file.

Create Digital Elevation Model: The wilderness breach single-beam and ground-based x,y,z data were imported into ArcGIS using the "create feature class” from the “xy table” tool in ArcCatalog. The dataset was then subsampled using the "subset features" and 10 percent of each dataset was removed to provide an estimate of the elevation uncertainty. The remaining 90 percent 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 25 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 (2015b), and any interpolated grid cells that were more than two cell sizes (50 m) away from a wilderness breach 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.

Evaluate Digital Elevation Model Uncertainty: The raster was sampled at the positions of the 10-percent-removed data using the "extract values to points" tool and a .csv file was saved from each data table for the personal watercraft and backpack data. Using Matlab (2015b), 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 20 cm.

Convert Raster Format: In ArcCatalog, a raster dataset was created using the "create raster dataset" tool with a .tif image format and 64-bit pixel type. The shoreface raster was then imported into this dataset using the "mosaic" tool.

Added keywords section with USGS persistent identifier as theme keyword.