Pacific Coastal & Marine Science Center
USGS Pacific Coral Reefs Website
SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) aircraft fly missions over shallow water shelves. Reflected laser light is detected, recorded, and later processed to make images like those shown below.
Lidar (light detection and ranging) is an airborne laser-ranging technique commonly used for acquiring high-resolution topographic data. The SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) system collects high-resolution bathymetric data in shallow, offshore areas. These maps essentially provide a view of seafloor topography. In areas with coral reefs, this includes not only the top surface of the reef but also associated channels and sand flats. Lidar is useful in shallow-water settings from near sea level to a depth of approximately 120 feet (35 m).
Maps created using lidar data reveal topography details of reef structure and morphology that are not available from aerial photographs. In addition, lidar systems can gather information from depths approximately three times greater than standard aerial photography.
For more information on mapping the South Molokaʻi reef tract using lidar data, see USGS Scientific Investigations Map 2005-2886 "Bathymetry and selected views of the fringing coral reef, south Molokaʻi, Hawaiʻi".
SHOALS imagery (above) of the South Molokaʻi reef tract shows the position of a distinctive channel that crosses the reef and a large hole in the reef. The large hole is shown below in oblique view. The origin of features such as this one is being investigated by USGS scientists (see Blue Holes and Their Origin).