Lidar a viable tool for archeological exploration

PrecisionHawk’s Jaymie Young and Matt Tompkins fly a UAV with the hosts of History Channel’s “Lost Gold of World II” in the Philippines in late 2018. (Photo: PrecisionHawk)

The collection of about 300 giant geoglyphs known as Nasca Lines were etched into the ground in the Peruvian coastal plain 2,000 years ago by the Nasca culture in South America. They depict various plants, animals, and shapes and are so large that they can only be fully appreciated when viewed from the air. Unlike the Nasca Lines, most archaeological sites are hard to see from the air, especially those deep under thick jungle vegetation, as in the lowlands of northern Guatemala. Here, lidar’s ability to penetrate foliage makes it the ideal tool for archeological exploration. At the same time, the lack of infrastructure, including airports, makes UAVs the platform of choice for many such projects.

Challenges for lidar in archaeology

Operating aerial lidar for archaeology in a jungle environment presents four challenges. The biggest one is penetrating the canopy. “Typically, the sites that they have been exploring lately have been quite overgrown, and disguising what has been there for a long time, to the extent that even someone walking on the ground has not been able to see the scope of the site,” said Paul LaRocque, senior scientist at Teledyne Optech. Penetrating multiple layers of vegetation requires a minimum of two lidar returns. For heavily vegetated environments, Matt Tompkins, director of Flight Services at PrecisionHawk, recommends a five-return system.

The second challenge is ground-truthing the data collected, explained Thomas Garrison, assistant professor in Ithaca College’s Department of Anthropology. “Even though we digitally removed the forest, it is still there, of course.” For example, it is hard to confirm long roads between sites, because they run beyond a ground observer’s field of vision.

The third challenge is acquiring a GPS signal. “In the tropical environment, a UAV needs to return to its take-off site very precisely,” Garrison explained. Finally, jungles put a twist on the familiar trade-offs between fixed-wing and rotary-wing UAVs: “A fixed-wing UAV will cover a greater area, but you may not have enough space for it to take off where you want to fly your lidar and may not be able to recover it,” Garrison said.

Making lidar an effective tool for archaeology required the development of sensors with very high pulse repetition frequencies (PRF) and with multi-look angles, which better penetrate the canopy to reach the ground and any structures, LaRocque explained.

Manned aircraft vs. UAVs

Archaeology missions need aerial lidar sensors with sufficient peak power from each pulse to reach the ground after much of it is absorbed or reflected by the canopy. In this respect, manned fixed- and rotary-wing aircraft have an advantage over UAVs because they can carry larger sensors with a higher peak power. Plus, they can cover much larger areas per flight. “This is important because archaeologists have been finding that [areas of interest] are much bigger than they ever imagined,” LaRocque said.

Finally, forests and jungles make it challenging to find open areas for launching and landing UAVs and flying them by line of sight. “Where do you stand to see the UAV above the canopy?” For these applications, LaRocque said, manned aircraft probably fly just 500 to 1,000 meters above ground level, “because they are not out for high survey efficiency but for penetration to the ground.”

Tompkins, whose company has conducted many missions in jungle environments and other remote places, takes the opposite view. Archaeology constantly presents new challenge and “takes you to some pretty crazy places,” he said and UAV lidar “allows archaeologists to access information that would otherwise be too difficult or dangerous to access.” In the projects on which PrecisionHawk has worked in heavy jungle, it has seen “excellent penetration through heavy jungle canopy, which gave us access to ground-level data,” he said.

Logistically, Tompkins pointed out, UAVs are “significantly easier and safer than manned aircraft” in remote areas with little access to airport infrastructure, air traffic control, and any sort of emergency services. A team with a UAV can mobilize and travel quickly. Where there is no electricity to recharge the UAV’s battery, the team will bring a generator.

Positional accuracy

Archaeological applications do not require survey-level positional accuracy. Archaeologists only need to be able to locate on the ground features that they identify in the 3D lidar point cloud. “We use Trimble Juno 5s, so that we can load the lidar data directly onto their screens,” Garrison said. “If we get near a structure, we only need to take a point. We don’t even need to find base stations and do corrections afterward. We can manually correct the points based on where they were on the structures.”

Ancient structures are rarely in the areas with the thickest, nastiest vegetation, which are usually low-lying areas, Garrison said. However, those areas might reveal other items of cultural interest, such as agricultural fields. “In the lidar data we have, we are seeing that ancient societies were terraforming their entire landscape, so everywhere you go there is something to check or confirm.”

Missions

In 2018, PrecisionHawk conducted an archaeological project in a remote jungle in the Philippines for a History Channel show called “Lost Gold of WWII.” Show producers were trying to find where Japanese troops might have stored gold stolen from the Philippine government.The team flew lidar on a UAV to identify roads or other manmade structures that could help them identify possible hiding spots in areas pinpointed through historical documents, Tompkins recalled.The PrecisionHawk team produced a colorized lidar data file in the jungle that the producers used on the program.

The largest lidar acquisition for archaeological investigations in the world is the Pacunam Lidar Initiative (PLI), a consortium of scholars funded by the Guatemalan foundation Pacunam (short for Fundación Patrimonio Cultural Y Natural Maya). So far, the PLI has scanned about 7,000 square kilometers in the lowlands of northern Guatemala.

Garrison is one of three principal consultants for the foundation. “Lidar has a huge impact in archaeology under tropical forests because you get more bang for the buck in terms of peeling back the forest and revealing the preserved ancient landscapes underneath,” he said. “We’ve been working on these data since we first flew in 2016 with the National Center for Airborne Laser Mapping in Houston. We published preliminary results in Science in 2018.” In the massive PLI project, “Instead of just single sites, we can see broad trends across huge areas.”

The first phase of the Pacunam lidar initiative covered 2,100 square kilometers in northern Guatemala in 10 different polygons of data acquisition. “We found that there were 60,000 more structures in these areas than we knew about before,” Garrison said. This raised the question of how so many people could feed themselves. The lidar data revealed that they had used huge swampy areas as agricultural fields. It also revealed many road systems between and within these sites and many defensive earthwork features of military engineering for warfare distributed along the valleys.

“All of that changed our view of what this ancient landscape looked like,” Garrison said. In the summer of 2019, the project flew another 5,000 square kilometers and is now analyzing the data, which shows “a complete picture of an ancient culture’s impact on the landscape,” Garrison said. “When you think of ancient Rome and everything that they had, you can’t really see everything because modern Europe is there. But here we have all this stuff preserved under the jungle in a way that is letting us see the totality of an ancient culture.”

Discoveries made with UAVs

• Using UAV lidar, the Crow Canyon Archaeology Center and the Canyons of the Ancient National Monument were able to map an 800-year-old Pueblo site at Sand Canyon, Colorado, and discover previously undocumented structures. UAV experts from Caddis Aerial and Routescene conducted the survey using a Velodyne HDL32 lidar scanner on a DJI M600 Pro UAV flying at 40-meter altitude relative to the take-off point. They then processed the point cloud, consisting of 3.2 billion points, using Routescene’s LidarViewer Pro software and created a digital terrain module at 400 points per square meter.

• A hexacopter built with DJI technology, flying 20–60 meters above the ground, enabled Isabelle Heitz of AirD’éco to map several ancient sites, including the microtopography of an ancient theater covered by woods in the center of Soissons, a town in eastern France, and a fortified town of the Gallic age, now covered by thick woods.

• Using Google Earth, satellite imagery and low-flying UAVs, archaeologists Sarah Parcak and Christopher A. Tuttle discovered a previously unknown monumental structure that had been hidden in plain sight only half a mile from the World Heritage site of Petra, Jordan.