- Associated Press - Friday, November 27, 2015

CONCORD, N.H. (AP) - Even though New Hampshire is covered by trees, little details like cellar holes and old stone walls have been recorded with the use of new airborne technology.

It’s called light detection and ranging - better known as LiDAR - and it’s been used to map three-dimensional land surface elevations across the state to help with flood plain information and broadband reach.

Here’s how it works: An airplane flying 180 mph in a straight line at an altitude of over a mile directs a rapidly pulsing laser toward the ground. The laser sweeps back and forth, collecting data to achieve complete coverage, much like mowing the lawn.

A detector measures how long it takes for each beam of light to travel to the ground and reflect back to the plane. Half of the round-trip travel time is converted to distance based on the speed of light, resulting in millions of closely spaced ground elevation data points.

The data will help with flood mapping. James Martin, a spokesman for the Department of Environmental Services, said it has already helped Seacoast communities in estimating rising sea levels.

“We can use it to map headwater streams, it will help us to really be able to zoom in on those kinds of geologic elements that we weren’t able to before,” Martin said. “Forestry management will be another application. Another would be assistance for Internet broadband planning.”

The state has recently finished collecting data for more than 80 percent of New Hampshire. It will be available to the public by late summer or early fall 2016.

The data collection project is a partnership between the state, the U.S. Geological Survey, the USDA Natural Resources Conservation Service, the Federal Emergency Management Agency and the White Mountain National Forest.

It is part of a national project announced last year by the Obama administration to map the entire country using LiDAR. The goal is to have it completed in eight years.

Before LiDAR, topographic maps produced decades ago by the USGS were the most commonly used source of elevation data for engineers. But elevation contour lines on these maps have a vertical accuracy of plus or minus 5 feet for much of the state and plus or minus 20 feet in more mountainous areas, too inaccurate to support modern computerized mapping methods. LiDAR technology has been able to provide a vertical accuracy of plus or minus 6 inches.

The National Oceanic and Atmospheric Administration says LiDAR was first developed as a fixed-position ground-based instrument for studies of atmospheric composition, structure and clouds, and remains a powerful tool for climate observations around the world. LiDAR sensors also are put on fixed-position tripods and are used to scan specific targets such as bridges, buildings, and beaches for terrain-mapping. Today, it’s used in agriculture, archaeology, law enforcement and other areas.

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