Data Acquisition

Navigation

The horizontal and vertical positions of each single-beam sounding and ground-based GPS points were collected using a base-rover configuration, with data recorded at 10 Hertz (Hz) using Ashtech ProFlex 500 Global Navigation Satellite System (GNSS) receivers and Thales choke ring Antennas. Two fixed GPS base stations collected data continuously for each day of the survey on a benchmark near Robbins Rest, Fire Island, N.Y. (REST 654955.02 m East and 4500721.18 m North in North American Datum of 1983 (NAD83), see Henderson and others, 2015) and published National Geodetic Survey (NGS) benchmark U374 (Permanent Identification number (PID#) KU0206; 674917.88 m East–4508863.81 m North, NAD83) (fig. 1). The vertical offset between the benchmark and the GNSS antenna was 2 m for REST and 1.8 m for U374. Rover GPS data acquired by the PWCs and backpack were downloaded at the end of each day, and the base station’s GPS data were downloaded either daily or before the next day’s survey.

Single-Beam Acquisition

Nearshore bathymetry was measured using single-beam sonar systems and GPS receivers mounted on two PWCs (fig.5). Two PWCs were used: a 2010 and a 2013 Yamaha VX Deluxe. Each PWC was fitted with a single-beam transducer below the waterline off the starboard stern, 1.11 m beneath the GPS antenna position. A waterproof Pelican case attached to the stern of each PWC contained a Cappuccino personal computer (PC), a Teledyne Odom Ecotrac CV-100 Digital Hydrographic Echo Sounder, and an Ashtech Proflex 800 GNSS (fig. 5). The system was powered by a marine battery located in the front storage compartment of the PWC, and the user interface included a waterproof display, a waterproof keyboard, and a mouse.

Figure 5. Personal watercraft instrument configuration (left) and Pelican Case hardware configuration (right). [Click image to enlarge]

The sonar system used a 200 kilohertz (kHz) single-beam transducer and recorded raw, digitized acoustic backscatter returns with a vertical resolution of 0.01 m ± 0.1 percent depth. Transducers with 4° beams were used during the first survey day. Due to inconsistencies between PWC soundings and large variations in the data between adjacent soundings, the transducers were changed to 9° beams for the remainder of the survey. HYPACK, a navigation and data acquisition software package, was used to merge sounding into a raw data file (.raw) that contained the estimated seafloor depth and a sounding file (.bin) that contained the return intensity in real-time.

Vessel operators followed predefined track lines using a HYPACK map interface, which displays vessel position in real-time. Within the wilderness breach, planned lines were followed when possible, but the abundant shoaling required operators to follow deeper tidal channels. Additional soundings were collected within the main breach channel to obtain high-resolution control over channel boundaries in places where large depth changes occurred. Additional shore- parallel lines were collected at various locations along the Fire Island shoreface to estimate system uncertainty.

Ground Based Mapping

Elevation data were collected using an Ashtech Z-Xtreme GPS receiver mounted in a SECO backpack using Ashtech Marine antennas attached to a pole that extended above the head of the surveyor. The relative elevation of the antenna to the ground was measured for each surveyor in a walking-stride position that varied between 1.67 and 1.7 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 (fig. 6).

Figure 6. Flood shoals and shallow channel elevation data collection using a backpack Global Positioning System within the wilderness breach. [Click image to enlarge]

Sound Velocity

Accurate sound velocity measurements are required to correct sounding errors due to density gradients throughout the water column. Water column sound velocity measurements were collected periodically throughout the survey using a SonTek CastAway conductivity, temperature, and depth (CTD) system. Data were processed using SonTek CastAway CTD software version 1.5.