Katharine Johnson remembers coming upon the distinct features while playing in the woods as a child.

“I always associated them with forests,” said the Rhode Island native, who is now a Ph.D. student in the Department of Geography at the University of Connecticut. “But of course they’re really an indication that the land was once cleared.”

If you have ever set foot in the New England woods, or even driven along a back road, you probably know what she is referring to: the ubiquitous walls of stone that meander through woodlands, tracing former boundaries of fields, orchards and farmsteads.

Although these features are common, their significance is difficult to comprehend. During the past century, maples, beeches, birches and hemlocks have reclaimed the landscape, obscuring the evidence of how drastically the early settlers reconfigured the land. It’s impossible to understand the magnitude of the impact without removing the trees.

Johnson and her advisor, William Ouimet, have figured out how to do just that without breaking as much as a twig. Light Detection and Ranging (LiDAR) is a remote-sensing technology that uses laser scanners mounted on airplanes, coupled with sensitive Global Positioning Systems (GPS), to take precise measurements of the ground surface. As a plane flies over the landscape, the scanner emits millions of laser pulses that reflect off whatever is below — trees, buildings, even birds.

“It measures the return times of all of these laser pulses, and because the GPS provides the exact location data, you can place those points in a precise alignment,” Ouimet said. The result is a three-dimensional image of the land surface.

Although LiDAR has been around for decades, conducting these surveys is prohibitively expensive. But Ouimet and Johnson are using publicly available data collected through the Northeast LiDAR Project and the USDA Natural Resource Conservation Service to pilot a novel application for the technology.

“We are taking that cloud of points and subtracting the trees, buildings and birds, so we are left with only the ground surface,” Ouimet said. “As if producing a high-resolution topographic map.” It’s a technique that has been used for archaeological reconnaissance in parts of Europe and South America, but never in New England.

In a seminal article published in the March issue of the Journal of Archaeological Science, he and Johnson share findings from three sites where they had access to both LiDAR imagery and historical records — Westport, Mass., Tiverton, R.I., and Ashford, Conn. — to demonstrate the power of this technology.

The improvement in quality can’t be overstated. Before LiDAR, the most sophisticated maps in the United States were created using stereo aerial photography, in which each pixel represented a 10-meter-by-10-meter area. By contrast, each pixel in a LiDAR image represents 1 square meter of ground — a tenfold increase in resolution.

“It’s basically at the scale as if you were out walking in the field,” Ouimet said. “Some say it’s better, because you can see subtle features that might disappear at close range.”

A physical geographer who studies processes that shape the landscape, Ouimet said the technology could revolutionize his research. “Not only do you have the high-resolution image, but you have the ability to move around a vast landscape very easily on a computer,” he said. “You could never cover that much ground on foot.” Especially if that ground was inaccessible, remote or privately owned.

While the technology will make it easier for Ouimet to do some of the work he is already doing, he said it’s also expanding the horizons for research. “It’s leading to areas that I never even knew warranted investigation.”

Treasure trove
Stonewalls aren’t the only features emerging from the shadows.

“When we got our hands on the data, we immediately realized we could see things like foundations and mill dams — old structures that might be as small as a couple of feet in width, yet have a clear expression in the imagery,” Ouimet said.

The findings have continued to amaze them. “Time and time again, we are looking at large tracts of forested land, and when we remove vegetation and look at ground surface, we are just finding a treasure trove,” he said.

Among those treasures: individual farmsteads.

Part of what makes these features so captivating, and important, is that the shapes are unexpected. While farms typically have rectangular footprints today, earlier farmsteads exhibit irregular polygonal patterns that Ouimet said, “Just pop out at you.”

It’s a surprise that is generating interesting new questions about the early settlers, such as, “When they started to break up the land and move stone, what were the physical rules that governed why this shape emerged?”

Geology was the determining factor, evident in patterns that Ouimet and Johnson have identified in the images. They found one farmstead situated on top of a hill surrounded by arable land, with no walls, and others that seem to have been developed on islands of topography in and around wetlands. “So the quality of the land regulated how farmers shaped the land, and how they set the boundaries around it,” he said.

But the rules changed with the passage of time. “It looks like these irregular polygons are associated with the initial settlement period,” Ouimet said. “As time progresses, you see more rectangular forms emerging.”

As competition for space grew, uniform parcels and boundaries became a necessity. As Robert Frost observed: “Good fences make good neighbors.”

History in focus
“I knew they were out there, but being able to see them at this resolution, and at this scale, all of this is a surprise to me,” Johnson said.

The New England landscape was already familiar territory for Johnson when LiDAR came into the picture. During her master’s program in historical archaeology at the University of Massachusetts Boston, she investigated landscape stability in Little Compton, R.I. a small community that had experienced little change in population during the industrial period, when the region as a whole was undergoing a dramatic transition.

For her thesis, she described the settlement processes that created the rural landscape, and analyzed first-person accounts that articulated a common desire to preserve the “charm of remoteness” that characterized the small, farming community.

When Ouimet contacted Johnson three years after she finished her thesis, describing the newly available LiDAR data and asking if she would be interested in working with him, she recognized the possibilities immediately.

“It’s the perfect tool to answer the kinds of questions I had been trying to answer,” she said. The visible traces of human settlement revealed through LiDAR would make it possible to corroborate the patterns suggested by historical records.

Although Little Compton was able to resist industrial pressures thanks to its proximity to affluent port cities such as Newport and Providence, more isolated farming communities in the region didn’t withstand the economic shift.

“In a lot of cases, especially in northeastern Connecticut, the process of industrializing led to farmstead abandonment, so the forests just regrew,” Johnson said.

Although people have conducted studies on lost farmstead features through archival research, Johnson said physical evidence could help answer questions about their lasting impact on the landscape.

Robert Thorson has been thinking about these kinds of questions for three decades. In the mid-1980s, he moved to Connecticut from Alaska to take a position in the Geology Department at UConn, and was instantly fascinated by the ubiquitous stonewalls. As a geologist and an archaeologist, he said, “They just took me in.”

His fascination eventually led to a 2002 book exploring the relationship between stonewalls and local geology. To his surprise, it was a hit. “Stone by Stone: The Magnificent History in New England’s Stone Walls” was a best seller, and an instant classic.

Thorson’s argument: More than just remnants of a bygone era, stonewalls are the products of human ecology. They are an enduring indication of the drastic alterations European settlers made to the environment in order to survive. While trees have reclaimed thousands of cleared acres, the walls persist — geology permanently reconfigured.

Although the magnitude of this impact is difficult to fathom, the lengths is estimated to be about 250,000 miles, Thorson said. “If you think about it, it’s ridiculous. Particularly given the scale of our landscape.” If placed end to end, the stonewalls of New England would circumnavigate the Earth 10 times.

“It is a colossal mass,” he said. “This was human beings doing their thing, and stonewalls occurring as a consequence.”

He said Ouimet and Johnson’s paper provides critical perspective on how humans had an enormous impact on the region, both physically and culturally. “Their paper brings it all home by showing the extent to which these features exist across the landscape, hidden under the trees,” he said.

Applications for conservation
Scholarship aside, Johnson believes it’s important for the public to understand the significance of the region’s agricultural heritage. For New Englanders, encountering stonewalls in the middle of the forest is a shared experience that contributes to a shared sense of place.

From a preservation standpoint: “If we can educate people about the kinds of sites that are out there, they can help us record and preserve our collective history,” she said.

So far, people seem eager to learn.

“This could change what you think you know about an area,” said Mike Shand, vice president of the Tiverton Land Trust, a volunteer organization established to protect scenic woodlands and rural areas in the small Rhode Island community.

“You think you know something about history generally from local stories, but what’s really there is another thing,” he said.

In partnership with the Sakonnet Preservation Association, the land trust has invited Johnson to come to Tiverton to present her research April 30, and Shand said he expects the findings could inform the way they manage the 450 acres within their care. From designing new trails, to placing new signs, to exploring new areas, “we want to find out what should we be doing.”

Identifying features is just the starting point. Ouimet said they want to extract more information, such as the height of the walls and depth of foundations, and do field validations of these kinds of measurements, to see how close they can come with the LIDAR.

As LiDAR is helping to shed light on the extent of these features, the features can then be used to validate the technology.

For Thorson, things are coming full circle. He has discussed the potential for collaboration with Ouimet and Johnson, and said he was already planning to revitalize the Stone Wall Initiative, a project he established to promote investigation, appreciation, conservation and education in this arena.

“I have been working on those goals, but have not done much with investigation,” he said. “That is where there has been a void.”

This new approach promises to fill the void. “You can know something is there, but seeing it makes all the difference,” Thorson said. “LiDAR allows us to see what we know has been there all along in such a clear, indisputable way.”