Ground-Based Bathymetric Data Collected within the Wilderness Breach, Fire Island, New York, (2014) in XYZ ASCII Text File Format

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.

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 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 line, along pre-defined transects, and additional data on the subaerial beach.

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.

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.

Extract Ground-Based Wilderness Breach 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 within the Fire Island wilderness breach. 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.

Added keywords section with USGS persistent identifier as theme keyword.