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New Maps from Old Photos: Measuring Coastal Erosion in California
USGS scientists and their coauthors from the California Coastal Records Project have found a way to use historical aerial photographs not just to see evidence of coastal erosion, but to accurately measure how much has occurred over time. Applying a low-cost technique called “structure-from-motion” to five sets of oblique aerial photos spanning the years 2002 to 2010, the scientists measured the volume of material eroded from coastal cliffs at Fort Funston, on the westernmost edge of San Francisco.
Quantifying coastal change is essential for calculating trends in erosion, evaluating processes that shape coastal landscapes, and predicting how the coast will respond to future storms and sea-level rise, all critical for communities along the California coast.
Kenneth and Gabrielle Adelman of the California Coastal Records Project photograph the California coast every few years and make the imagery publicly available online. “These photos are not only great for looking at the changing shoreline,” said USGS geologist and lead author Jon Warrick, “but, as we’ve now shown, they can also be used to build 3D maps of beaches and cliffs. And these maps enable us to measure coastal change.”
To measure coastal change in central California, Warrick and his USGS team began shooting structure-from-motion photographs from a chartered airplane. They wondered if they could apply the technique to existing, older photos to extend measurements back in time. They gave it a try with California Coastal Records Project photographs of cliffs along a half-mile stretch in Fort Funston, a part of the Golden Gate National Recreation Area known to have relatively high erosion rates. The effort succeeded, yielding many details about the timing and magnitude of changes over the 2002–2010 study period, including landslides, rock falls, and the buildup and erosion of landslide debris on the beach.
“We are thrilled about this new use of our photos,” said Gabrielle Adelman, who flies a helicopter along the coast while her husband Kenneth shoots photos. “The photos are a lot of work to take, and we are glad that they are so useful in so many ways. I trained as a scientist, and it’s good that so much useful data have come of them.”
The California Coastal Records Project is “an exceedingly valuable resource,” said Gary Griggs, an authority on California coastal change and director of the Institute of Marine Sciences at the University of California, Santa Cruz. Griggs has worked with the Adelmans and helped facilitate their collaboration with Warrick. He said, “I’m really excited that Jon was able to connect [with Kenneth and Gabrielle] and make this analysis of historical photographs work out as a new tool that I hope others use as well.”
Because helicopter time is expensive, Kenneth Adelman takes photos that overlap, so that if one is flawed, adjacent photos can provide much or all of the missing data. Such overlap is essential for structure-from-motion analysis, a relatively new technique for using two-dimensional images to create 3D maps. The structure-from-motion software picks points that can be identified on two or more overlapping photos—the corner of a building, for example, or the tip of a distinctive rock—and uses triangulation to measure their relative positions in space. Combining points from numerous photos produces a 3D image, sometimes called a “point cloud.” By surveying some points with GPS or a Global Positioning System, the mapper can assign a geographic position to each point, making the 3D image a 3D map.
Warrick said, “We can produce about 20 to 150 mapped points per square yard from the California Coastal Records photos,” which added up to millions of points across the study area. Processing so much data requires considerable computing power, one reason the structure-from-motion technique has only lately come into wider use.
To test the accuracy of the Fort Funston structure-from-motion maps, the scientists compared them with maps produced by airborne lidar (light detection and ranging), an established technique that uses a laser scanner to measure distance from the instrument to various points on the ground. The structure-from-motion maps were found to have accuracy similar to that of the lidar maps, and they had the added benefit of imaging vertical and overhanging sections of cliff that could not be “seen” by airborne lidar. Because structure-from-motion relies on digital cameras rather than a laser scanner, it is much less expensive than lidar. Applying it to existing aerial photos makes it even more cost-effective.
“The success of our study shows that imagery originally obtained to provide qualitative assessments of our coasts can now be used in quantitative assessments of coastal change,” said Warrick. The techniques reported in the new paper might be broadly applicable to photo sets of coasts throughout the world.The results were published October 21, 2016, in the Journal of Coastal Research (“New Techniques to Measure Cliff Change from Historical Oblique Aerial Photographs and Structure-from-Motion Photogrammetry”).
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