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Mapping Techniques: Satellites and Aerial Photos

Hydrographic LIDAR

Illustration of LIDAR

LIDAR system in operation.
Courtesy: NOAA Coastal Services Center

Hydrographic LIDAR (LIght Detection And Ranging) has been used successfully for detection and mapping of bottom topography (that is, bathymetry) in inshore areas. The LIDAR sensor is mounted on a helicopter or fixed-wing aircraft and emits a narrow, high frequency laser beam. The system includes a transceiver consisting of the laser, scanner, and receiver that transmits laser pulses approximately 400 times per second and receives return signals. An aircraft positioning and auxiliary sensor collects position information from a Global Positioning System (GPS) receiver and inertial reference system and provides a video image of the area being scanned. The acquisition, control, and display system (ACDS) provides an operator interface to monitor and control the airborne system.

When the laser light beam hits a column of water, part of the energy is reflected off the surface and the rest travels through the water column and reflects off the seafloor. The water surface reflects energy from the infrared pulse, while the blue-green pulse is the one that penetrates the water column and is reflected off the seafloor. The system records the time it takes for the reflected signals from the surface (infrared) and seafloor (blue-green) to return to the aircraft. The water depth is calculated from the time difference between the return pulses.

Example of LIDAR

A bathymetry map based on LIDAR data from St. Lucie Inlet, Florida. Land areas appear white.
Courtesy: U.S. Army Corps of Engineers

Advantages and Limitations

Hydrographic LIDAR is extremely useful for regional coastal mapping. LIDAR systems can provide uniform and dense data in extremely shallow water. It is a good complement to acoustical surveys, which are less effective in depths less than about 5 meters. The biggest limitation of LIDAR, as with other airborne techniques, is its dependency on water clarity. In clear waters it can be used to depths of over 50 meters (over 150 feet), but in turbid water it is only successful to depths of two to three times the visible depth. LIDAR is cost effective for surveying large, shallow areas with generally good water clarity.

Uses

LIDAR provides high density, accurate bathymetric data for a wide range of applications. These include navigation projects, beach monitoring, and coral reef damage assessment. Some projects that have used LIDAR include: monitoring channel depths and dredged material disposal sites, nautical charting, and evaluating conditions of jetties above and below the water. LIDAR data have also been used to create wetland inventory maps and to monitor coastal erosion.

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