USACE Field Research Facility
Field Research Facility
Coastal & Hydraulics Laboratory

LARC Survey | Vehicle | Equipment | Analysis and Software | Photos

The LARC is uniquely designed to allow surveying in the water, across shoals, and even through the surf zone up to the base of the beach dunes.

Previous boat-mounted survey systems were unable to filter out wave motion, and were therefore limited to vertical accuracies of 30 cm (1 ft) and had to also adjust for changing tides. Our new system overcomes both of these problems by using a Real-Time Kinematic Global Positioning Satellite (RTK-GPS) survey system to determine the location and movement of the LARC. This type of system is being used on a number of different floating platforms, including jetskis, but the LARC offers the unique ability of being able to operate on the beach, over shallow shoals, in breaking waves, and offshore.

The LARC Survey System consists of the LARC and:

  • Trimble RTK-GPS for horizontal and vertical positioning of the LARC;
  • Knudsen 320B/P Echosounder to measure the depth;
  • Coastal Oceanographics HYPACK Hydrographic software for data logging and navigation;
  • Data collection laptop computer;
  • CTD (conductivity, temperature, depth) profiler to determine the speed of sound in water;
  • Data processing software that creates the X,Y,Z data file.

    In operation, the pilot steers a pre-defined course of survey lines over the area to be surveyed at a speed of about 7 km/hr (4 knots). The navigation system provides real-time position information to a computer monitor in the LARC cab which enables the pilot to accurately steer the line. A typical line begins at the foot of the dune and extends offshore to a predetermined depth. Since the echosounder data are affected by density changes in the water column, speed of sound profiles are collected periodically during the day with the CTD. While the LARC can operate under most wave conditions, air entrainment in the surf zone limits surveying to conditions with waves generally less than 1 m (3 ft).

    Survey Comparison

    A major activity of the Field Research Facility (FRF) is to conduct nearshore surveys, both at the FRF and at other coastal sites. At the FRF, the CRAB was the primary platform for surveys and the data collected with it since 1981 set the standard for high-accuracy nearshore surveys. Vertical accuracies of 2 or 3 cm (0.1 ft) are required to resolve small, but significant changes to the ocean bottom. While this is obtainable with the CRAB, the CRAB is limited to use at the FRF and to depths shallower than 9 m (30 ft). Since 1997, the FRF has been developing a new system using the LARC which can operate most anywhere, and which can obtain accuracies similar to the CRAB (see comparison below).

    The LARC web pages were designed and created by Justin Vandever (Contract Student, Cornell University)
    with input from Bill Birkemeier, Dan Freer, and Carl Miller. July 2001.