March 23, 2012 — The days of slogging through underbrush to measure ground-level elevations are over.
A team of scientists including John Porter, a University of Virginia research associate professor of environmental sciences in the College of Arts & Sciences, in collaboration with The Nature Conservancy, the U.S. Geological Survey and the Virginia Information Technologies Agency, recently used an innovative light detection and ranging technology, called "LiDAR," to complete a detailed elevation map of the Delmarva Peninsula and the barrier islands off Virginia's Eastern Shore.
LiDAR follows a concept similar to radar, except instead of radio waves, it uses light. To collect elevation data, laser pulses are sent downward from the belly of an airplane. A sensitive instrument then records how long it takes the pulses to bounce back to the plane.
It's as if you threw a handful of Ping-Pong balls against a wall and used a stopwatch to record how long the balls took to get to the wall and back, Porter said. If you are close to the wall, the balls will return quickly; if far from it, they return more slowly.
While older forms of LiDAR were impaired by shrub growth, the more contemporary processed data can distinguish ground cover and buildings from the surface. Complications do still arise from flight conditions –clouds can interfere with the lasers' path, and sudden plane movements such as from hitting an air pocket can adversely affect the data.
"LiDAR is a way of capturing a huge amount of data over large areas with a high degree of accuracy and a high rate of speed," Porter said. From the tip of the peninsula to the Maryland state line, 95 percent of the points on Virginia's Eastern Shore were vertically within 7 inches of where they should be. This type of data is so advanced that analysis methods are still catching up. "It's a tremendous amount of data and right now really difficult to analyze," he said. "It's the type of data we've never had access to before."
Porter and other members of the Long-Term Ecological Research Network, or LTER are using LiDAR as they seek to understand what drives the patterns of vegetation on the barrier island landscape.
"The big thing that tends to structure the system is elevation, in the sense that when you go from the grassland into a shrub thicket … you've gone up a little higher into an area that isn't flooding as frequently," Porter said. "The differences can be very subtle – six inches or less – to have a big impact on what the type of vegetation is."
Without the LiDAR elevation data, the team has difficulty distinguishing the effects of elevation from other contributing factors. In this environment, the surfaces of land, saltwater and the fresh water table all interact. Elevation plays an important part in the composition of the island salt marshes, which are experiencing a sea-level rise of 4 millimeters a year, or roughly 1 foot per human lifetime. This represents a "pretty darn high" rate of change that could completely alter the composition of the marshes and potentially endanger the habitat, Porter said.
Dana Oster, a student in the environmental science's master's program in geology is using LiDAR to create inundation maps to predict sea-level rise scenarios. Oster has been finding that recovery of beach shore erosion is most closely linked with regaining dune elevation. This research helps explain how an island recovers in a natural state, which has implications for development; her research shows that on a narrow island, a house or sea wall in place of a dune prevents the beach from restoring.
The Long-Term Ecological Research team leveraged its funding, through partnerships with The Nature Conservancy and the U.S. Geological Survey, to obtain data not just for the barrier islands, which are largely uninhabited, but for the whole peninsula, which Porter said helped two of the poorer counties obtain the same coverage as wealthier localities that can afford to fly LiDAR-equipped planes more regularly.
This data has a lot of value for the residents of the Eastern Shore. "If you wanted to locate where you were going to build a house, knowing what the elevation is is critical," Porter said.
Additionally, the data helps local planners weigh possible future scenarios. "Do you want to build a school that's going to have to be moved in 50 years because the water level's going be too high, or do you want to build it someplace where it's not?" Porter said.
The LiDAR data is freely available to the public. It may be incorporated into Federal Emergency Management Agency flood maps, which will affect house insurance premiums. In the future, the data will also be used by the National Oceanic and Atmospheric Administration and incorporated into detailed storm models and evacuation plans. "They're going to be able to be much more precise about which areas need to be evacuated," Porter said.
– by Kate Colwell