for National Geographic News
Criss-crossing the sky in a coastline-hugging pattern, the blinking aircraft fires a flurry of green laser light into shallow waters below. Over the next seven hours, more than six million laser pulses will be transmitted into the water as the low-flying plane completes its mission.
A futuristic invasion? A military war game? Neither. Actually, the fixed wing aircraft is engaged in old-fashioned nautical sea chartingwith a high-tech, high-flying twist.
The twist is airborne lidar bathymetry. Bathymetry is the measurement of water depths. Airborne lidar bathymetry (ALB) is a way of measuring water depth from the air, using lidar. Lidar works like radar but substitutes laser pulses for radar's microwaves.
Faster, more accurate, more environmentally friendly, and cheaper than traditional, boat-based methods, ALB could go a long way toward mapping the literally millions of uncharted coastline around the world. Used by governments, private companies, and military planners to chart coastal waters as deep as 230 feet (70 meters), it has emerged as an increasingly important tool in recent years.
How It Works
Here's how ALB works: The lidar fires a laser pulse from the aircraft toward the sea. The laser is reflected at both the water's surface and the seafloor. ALB equipment registers those two reflections.
The amount of time that passes between the two reflections determines the water depthtaking into account the speed of light in water and variables such as ocean waves and tides. This process repeats millions of times during a survey.
The result? A map of the waters below that can be used for navigation, oil and gas exploration, national defense, economic-zone demarcation, and beach and reef management.
"It may be hard to believe that you can get a good map of the waters from the sky," says Mark Sinclair, a pilot with Australia's Tenix LADS Corporation, which is a leader in ALB. "People don't believe until they see it in their backyard," he says.
Veteran hydrographer (or water mapper) Jeff Andrews, originally a skeptic, proclaims himself "a believer." Andrews says he was "amazed by the results from the air survey" conducted for his firm, Coastal Planning and Engineering, Inc. The firm used the results for beach-restoration projects and environmental studies.
Thus far, ALB mapping has proved comparable to ship-based mapping in accuracy. ALB still cannot, however, match the most precise form of ship survey used for heavily trafficked coastal waters. But ALB can produce surveys up to 20 times faster than ship-based systemsand at roughly half the cost, advocates say.
Says John Longneck, an ALB expert at the U.S. National Oceanic and Atmospheric Administration (NOAA): "As the need to chart large areas of coastline grows, ALB has become an essential tool. ALB is fast, cost-effective, and most importantly, safe. Airborne hydrographers, unlike ship-based hydrographers, are operating free from the rocky, treacherous coastline and harsh ocean environment."
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