Eye in Sky Can See Underwater Greenery

Under a searing summer sun, an NC State research team takes a small boat out onto Currituck Sound and drops some lines in the water. This is no ordinary fishing expedition, however. The group is reeling in samples of plants growing in the sediment a few feet below the surface so they can correlate their findings with images snapped by a satellite soaring more than 275 miles over their heads.

Determining the location and variety of vegetation submerged in a body of water has always been a labor-intensive process. It's also one with the potential for inaccuracy, as researchers pull samples from various points and then extrapolate their findings over a wider area. The North Carolina Department of Transportation (NCDOT) asked Dr. Stacy Nelson, associate professor in the Department of Forestry and Environmental Resources, to find an easier and more reliable way to inventory underwater plants. Submerged vegetation often presents planning problems for NCDOT engineers on projects that cross water. They must either avoid areas teeming with plants because those spots often are also teeming with fish or mitigate the construction damage by replanting elsewhere. "If you don't have an accurate inventory of what's there," Nelson says, "you don't know where to avoid and what to mitigate."

Nelson, who works in the College of Natural Resources' (CNR) Center for Earth Observation, believes high-resolution satellite images can be used to pinpoint submerged vegetation. He says he had good results using satellite photos to map lakes in Michigan a few years back, and now he is tapping into more detailed photos from a commercial satellite called Quickbird to develop a mathematical model for predicting what plants are where underwater. The model also will include data on water quality and the "reflective signature" of various plants.

Submerged vegetation often presents planning problems for NCDOT engineers on projects that cross water.

Getting the signature for each species is where the boat trips on Currituck Sound come in. "Water either scatters or absorbs the reflective energy the satellite is trying to capture, so there's little left to develop a signature," Nelson says. Together with Dr. Tom Colson, a geographic information systems expert in CNR, and two graduate students, he had to conduct a manual inventory so they could match their findings to the Quickbird photos. They collected samples at 276 points over 270 square miles three times during the summer as vegetation changed, noting the global-positioning satellite coordinates of each point so they would sample the exact spots each time. "If we can use the model to preserve healthy plant communities," Nelson says, "we can boost the underwater ecology along our coast."


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Dr. Stacy Nelson and Brett Hartis, a Ph.D. candidate in Forestry and Environmental Resources, examine underwater vegetation in the Currituck Sound.