by Mary Caperton Morton Tuesday, August 19, 2014
In recent years lidar has become the gold standard for people looking to make high-resolution aerial maps — from archaeologists studying ruins hidden beneath jungle canopies to engineers monitoring dams and levees. Although the technology has many useful applications, it’s often prohibitively expensive. Now, a new technique using an off-the-shelf digital camera is offering an inexpensive alternative for collecting 3-D aerial data.
Over the past 10 years, the John Day River in central Oregon has undergone extensive restorations to support the river’s wild salmon population. “We wanted to image the whole river valley to help track the restoration efforts,” says James Dietrich, a geomorphologist at the University of Oregon. “But lidar would have been much too expensive.”
So Dietrich instead mounted an off-the-shelf digital SLR camera on a helicopter and shot high-resolution images of a 29-kilometer-long section of the Middle Fork of the John Day River from Bates State Park to Galena, Ore. “We covered the whole river with 1,400 photos in about two and a half hours,” he says.
Dietrich then used an imaging technique called Structure-from-Motion to process the images into a 3-D digital surface model with resolutions down to 10 centimeters, much higher than resolutions typically achieved with lidar, which typically deals with resolutions down to 1.5 meters. “Photogrammetry [making precise measurements from photos] is not a new concept, but this is a new method,” he says. “We really had no idea if it would work but we ended up with some really great imagery that’s perfect for studying watershed restoration.”
“The resolution is very impressive,” says Noah Slovin, a hydrogeologist at the University of Massachusetts who was not involved in the new research. “The great thing about this method is that it’s inexpensive enough that you can image an entire watershed every few days or weeks or years to see how the stream is changing over time. With lidar, you’re lucky if you get new data every five years, which isn’t all that helpful when you’re studying something like stream restoration.”
The new method, presented by Dietrich at the Association of American Geographers annual meeting in Tampa, Fla., in April, still needs some work as questions remain about the vertical accuracy of the imaging. “We have two lidar datasets to compare our data with and the horizontal accuracy is spot on, but the vertical component is not matching up as well as we’d like,” Dietrich says. “I think it’s likely a software issue, but we’re still investigating.”
Ultimately, Dietrich hopes the method may help make aerial data more accessible to researchers. “Aerial photography is a significant investment in both time and money, no matter how you do it, but this low-cost, efficient and still effective method may be the key to making such imagery more available to more people.”
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