Data Processing

Navigation Data

Horizontal and vertical coordinates associated with each sounding were post-processed using differential corrections derived from the base/rover setup. Two GPS reference stations, located at benchmarks U374 and REST, were used for the survey. Reference station coordinates were verified with Continuously Operating Reference Stations (CORS) using the Online Positioning User Service (OPUS http://www.ngs.noaa.gov/OPUS/). U374 used reference stations ZNY1, NYRH, and CTDA, while REST used stations NYCI, NYRH, and MOR6. OPUS computed that both reference stations had a horizontal error of 0.4 cm and vertical error of 0.2 cm. After applying the reference station coordinates, GPS data acquired from the rover were processed to the concurrent GPS session data at the base station with GrafNav software (Waypoint Product Group) version 8.5. The horizontal and vertical coordinates were recorded in the World Geodetic System of 1984 (WGS84) reference frame and exported as an American Standard Code for Information Interchange (ASCII) file. Ground-based trajectory data were converted within GrafNav to North American Datum 1983 (NAD83(2011)) and North American Vertical Datum of 1988 (NAVD88, GEOID12A) and then exported as an ASCII file.

Single-Beam Data

Single-beam soundings were merged with differentially processed GPS data and sound velocity profiles using Matlab. Each transect was visually inspected for elevation outliers and dropouts associated with wave breaking in the surf zone were manually corrected. Typically, the highest intensity return is generated by the seafloor surface. Breaking waves in the surf zone can create air bubbles in the water column and create an erroneous peak in waveform intensity, which causes errors in the interpreted seafloor reflection. When this situation was 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 corrected for the average speed of sound (table 1). A moving average filter was applied to the soundings to reduce instrument noise and the noise associated with the pitch and roll of the PWC. The depth soundings (from the transducer to the seafloor) were then adjusted to the depth from the GPS antenna and subsequently to the WGS84 ellipsoid; referencing to the ellipsoid reduces error associated with determining mean water level or tidal elevations. NOAA’s VDatum software converted the sounding positions and elevations to NAD83 and NAVD88 (GEOID12A), respectively.

The accuracy of the single-beam soundings was evaluated by identifying locations where survey track lines crossed and soundings from each line were horizontally within 0.25 m of each other. Any track line with an error greater than 0.6 m along multiple crossings was removed. Evaluation of the track line crossings not removed indicated an overall mean difference of 0.7 cm and a root mean square (RMS) error of 12.2 cm for a total of 732 crossings. Since the mean error between wave runners was 0.5 cm, which is below the minimum resolution of the echosounder, no correction offset was applied to the individual echosounders. There is a total of 5.4 cm error in the transformation using the conversion from the WGS84 ellipsoid to NAVD88 (GEOID12A) (http://vdatum.noaa.gov/docs/est_uncertainties.html). Applying the square root of the sum of the datum conversion uncertainty and the sounding uncertainty resulted in a combined vertical error of 13.3 cm. Horizontal uncertainty is assumed to be half of the vertical uncertainty (6.7 cm) at most.

Ground Based Data

Erroneous ground-based horizontal and vertical positions, such as when the backpack was removed and when the surveyor was transported between shoals, were removed. Ground-based GPS elevation errors were calculated by computing the vertical differences at crossings that occurred at least 1 minute apart. An RMS error of 24.9 cm was calculated based on 210 crossings. Elevation differences between the ground-based and single-beam data points (487 samples within 50 cm) indicated the ground-based elevations were 6 cm higher than elevations recorded using PWCs. Given the high degree of uncertainty arising from of the backpack surveyor striding over a subaqueous surface, the ground-based data were adjusted to the single-beam elevation at the crossings.

Digital Elevation Model

The fully processed and corrected bathymetric data points (x,y,z) were separated into two regions; the wilderness breach and the shoreface; with the wilderness breach ebb-tidal delta located in both regions. The shoreface data points consist solely of PWC collected data and the wilderness breach data points include both PWC and backpack data. The bathymetric data points were exported as ASCII files from Matlab and converted to an ArcGIS shape file using the “create feature class” function from the xy table tool in ArcCatalog. These point shapefiles were used to create a triangulated irregular network dataset (TIN) for the shoreface and the wilderness breach using the “create TIN” tool. A raster dataset was created from the TIN using a 50-m cell size with a natural neighbors interpolation for the shoreface bathymetry (fig. 7) and a 25-m cell size for the wilderness breach bathymetry (fig. 8) using the “TIN to raster” tool. Due to the removal of erroneous lines and an inability to survey certain lines in the field, some interpolated grid cells are more than 2 cell sizes away from a sounding for both the shoreface and the wilderness breach. When a cell was more than 2 cell sizes (100 m and 50 m for the shoreface and the wilderness breach, respectively) away from a data point, the cell was set to null and appears as a gap in the bathymetry. To determine the uncertainty associated with the rasters, we used the method of Lentz and Hapke (2011) and Lentz and others (2013): we withheld 10 percent of the data points when creating the TIN using the ArcGIS “subset features” tool. The raster was then sampled using ArcGIS tool “extract values to points” at the corresponding eastings and northings of the withheld data. An RMS error between the sampled bathymetry and the measured value was found to be 14.0 cm vertically for the shoreface bathymetry and 19.3 cm vertically for the wilderness breach bathymetry.

Figure 7. A 50-meter grid of June 2014 bathymetry of the shoreface of Fire Island, N.Y. [Click image to enlarge]

Figure 8. A 25-meter grid of June 2014 bathymetry of wilderness breach, Fire Island, N.Y. [Click image to enlarge]