Lidar archaeology shines a light on hidden sites


Striking discoveries in archaeology are being made possible by strong beams of X-rays, say researchers.

For the best part of 25 years, archaeologists Arlen and Diane Chase slogged through the thick undergrowth in the west of Belize in search of an ancient city whose details had been lost to the passage of time and the decay of the jungle.

The going was tough, often requiring a machete to clear a path through the dense vines and creepers that blocked their way. Over time, their perseverance paid off as their hand-drawn maps began to reveal long-forgotten parts of the massive Mayan city of Caracol.

But the more the pair found, the more they realized the extent of what remained uncovered. It would take several lifetimes, they figured, to reveal the true extent of Caracol. 

Then, in 2008, they got talking to a biologist colleague at the University of Central Florida where they worked. For years, he had been using airborne laser sensors known as Lidar (Light Detection And Ranging) to map and study forests and other vegetation. He suggested they give it a go.

So, in 2009, the pair packed away their machetes and hiking boots and took to the air in a twin-engine plane equipped with the technology. For four days they skimmed backwards and forwards over the tree-tops firing pulses of laser light at the ground below. A few weeks later, back at their lab in Florida, the pair got their first look at the results.

“I was completely astounded,” says Arlen Chase. “We had not expected the clarity that we saw in the imagery.”

“I am pretty sure we uttered some expletives,” Diane adds politely.

In less than a week, the team collected more data than they had in a quarter of century of hacking their way through the jungle.  Analysis revealed a host of previously undiscovered features, including several in areas that they had previously mapped on foot. It was a revelation.

Now, archaeologists around the world are beginning to embrace the same technique, flying aircraft over everything from Stonehenge to patches of scrub, in search of hidden treasures. The findings are already beginning to challenge conventional theories and change our view of the size and extent of ancient civilizations. But, while some say we are on the cusp of a new golden age of discovery, it is also beginning to throw up difficult questions about the disappearance of ancient civilizations.

Down under

Using technology in archaeological expeditions is nothing new. Techniques similar to those used in the offshore oil industry have been used for years by archaeologists on the ground to spot buried structures. Increasingly, archaeologists are using satellite photography with success, for example, increasing the number of structures in the Nile valley including 17 new pyramids. In August, two more were found using images from Google Earth. Radar has even been used, famously uncovering vast new areas of the vast Cambodian temple complex Angkor Wat.

But Lidar seems to offer several advantages. It is quick, relatively cheap and can be used to map large areas very quickly, particularly those covered in dense vegetation.

The technology has been used in a variety of ways over the last two decades, from gauging distances between cars in adaptive cruise control to mapping forest canopies and detecting the amount of aerosols in the atmosphere. But, no matter what the use, most modern Lidar systems are essentially the same. All shoot thousands of pulses of laser light and then use sensors to detect any reflections. By measuring the time it takes between sending out a pulse and measuring the light bouncing back, software can begin to build up a picture of the machine’s surroundings.  Using it in combination with GPS and other location technologies, it allows very accurate 3D maps to be built.

Crucially, some of the laser light is also able to penetrate vegetation. So, in the case of areas covered in a forest canopy, such as in Caracol, some of the pulses will hit the top of tree canopy, some the middle, others the forest floor. Software can then be used to remove the points above the ground, according to University of Alabama archaeologist Dr Sarah Parcak, who is not part of the Chase team but has used Lidar at other sites. This leaves a detailed “digital elevation” model of the hidden forest floor with the ability to pick out features as small as 20cm across.

“It is an amazing tool,” says Parcak. “You cannot use anything else in areas such as Central America to visualize Mayan ruins in a clear way.

The measurements are exact enough that the Chases were able to overlay the new map onto the painstakingly, hand-created maps of their site with an unexpected level of exactitude. But what really astounded them was the amount of detail they had never seen before.

Previously, they had mapped around 3.5 sq km of agricultural terraces on the site. The Lidar revealed more than 150 sq km more.  In addition, it revealed thousands of new buildings arranged around squares, 11 new waterways, more than 60 caves as well as clues that suggest there could be up to 1400 water reservoirs on the site.  All in all, one fly-over had radically increased the size of the ancient capital. Overnight it changed archaeologists’ perception of the site from a rarely-inhabited ceremonial center to a bustling city with a complex system of agriculture to support it.

Devastating conclusion

It is tempting put characterize this kind of revelation as a one-off success. But, Lidar’s success is not confined to Caracol. Further north, a team from Colorado State University has also used the technique in the Patzcuaro Basin, a region in the west of Mexico. The area was the centre of the Purepecha Empire – contemporaries of the Mayan and Aztec civilizations that have never caught public attention. They can be thought of as the people who stopped the advance of the Aztecs into San Diego and were also famous for their intricate metal work.

In 2007, Colorado State University professor Chris Fisher began investigating the area. That year, he and his team found some impressive treasures including an imperial treasury building, where the leaders kept their stores of hummingbird and macaw feathers, the dominant currency. A year later, equipped with handheld GPS units his team spent three months on foot mapping the area in search of other treats. But, what they uncovered surprised even them.   

At a spot in an ancient road that previous surveys had marked down as little more than a widening of the carriageway, the team began to uncover evidence of buildings. Lots of buildings. Over three months, the team of between 12 and 16 people unearthed evidence for more than 1,400 buildings. It seemed that the wide spot in the road was in reality a surprisingly large pre-Hispanic capital.

But it wasn’t until last year that Fisher and his team would know just how big. Equipped with Lidar the team flew over that spot recording 3,000 buildings in half the time it had taken them with ground surveys.

“When Lidar was first used at Angamuco we had no idea how large the area was that included buildings and structures, if it was even a city,” team member Professor Steve Leisz told the BBC. Perhaps more surprisingly the team also found a ball court for a Meso American game called pok-ta-pok, and pyramids, including one that Fisher had walked within 10m of the previous year. “That was a complete surprise,” said Leisz.

Finding new buildings and even cities is all very well and good - a new Machu Picchu or Chichen Itzas would be the crowning achievement of any archaeologists’ career. But, it goes without saying, most buildings and land modifications are rarely so dramatic. What is really important is who was in these buildings and how many. The more buildings, roads, wells, agricultural terraces and residential complexes are found, the higher the number of people that lived there.

Population estimates of the Americas at the time of European contact have been steadily increasing over the past decades as archaeologists have slowly found new sites and dug over existing ones. That has gradually overturned the image of the Americas as a vast unexplored, unpopulated wilderness. But Lidar surveys are now beginning to dramatically change our view, says Fisher.

“Widespread Lidar surveys will reveal a Mesoamerican landscape that was more densely settled, and an environment that was more pervasively modified, then previously thought,” said Fisher. Instead of a wilderness, here were two continents with vast populations, grand urban centres and widespread agriculture. But, perhaps more importantly, in revealing what life was like before the Conquistadors arrived in the 1500s, it also reveals the devastation that they wrought when they came into contact with native populations.

“Before, a 40% die off seemed implausibly high,” said Fisher, “now 80% seems more likely.”

‘Exciting times’

You may expect that dramatic findings and conclusions like this would mean the case for Lidar has been made. But not everyone is convinced by the laser revolution. Archaeologists like Rosemary Joyce, a professor of Mesoamerican archaeology at the University of California, Berkeley, and Russell Sheptak, a visiting scholar there, believe that the some of the proponent’s claims do not stack up.

“What I specifically reject,” says Joyce, “is the claim that Lidar is both faster and cheaper than other archaeological methods, if we are interested in understanding sites, not just discovering them.”

To really understand a site, you need boots on the ground, they say.

These are arguments that the Lidar community are familiar with, and have some sympathy with. Both the Chases and Fisher teams admit that cruising over the tree-tops in a plane does not totally supplant the need to get up close and personal with a site. Without their 29 years of experience at Caracol, the Chases admit they would not have been able to recognize what they were seeing through Lidar as quickly as they did. Instead, it is a tool that allows them to quickly zoom in on potential features of interest. It also allows sites to be mapped quickly, allowing them to be preserved from looters and development.

However, they take issue with arguments based on cost. Although they admit that Lidar can be expensive at face value – usually around $350 per square kilometer - they maintain it is still cheaper than traditional digs. For example, the Chases calculate the cost of Lidar per square kilometer is vanishingly small in comparison with the cost of travel, living on site, hiring workers, provisioning the dig and the thousand other misfortunes that an expedition to the jungle can encounter. Fisher, however, puts it more bluntly: “I don’t know how people can say it’s not cost-effective,” he says. “It saved us 10 years of research, for the cost of one season of excavation.”

Over the next few years, as with all developments in computing, Fisher expects to see that cost continue to fall and its use to sky rocket. “Ten years from now, this is going to be like radiocarbon dating,” says Fisher, referring to a standard technique now used by all archaeologists to date finds. “Lidar is going to be folded into your research program, a really basic thing you do to understand the questions you want to answer.”

Already, the teams have their eye on other areas where it could be used. For example, Arlen Chase believes it could help us to better understand settlement patterns along the Amazon, which can now only be glimpsed in satellite imagery on tree-cleared landscapes. He also believes it will also allow us to understand ancient African migrations and cultures, also currently obscured by forests. Other targets include Sri Lanka, India and other sites around South-East Asia. They also believe it could begin to be used to find sites such as ancient harbours, currently covered by water.

“I suspect that, as we examine Lidar for different places, we’re going to wind up finding things in different places that we would not have thought to try to find,” says Leisz. 

Parcak puts it more prosaically. Technology, she says, is going to make us realize how little we know and how much left there is to explore. 

“It's the most exciting time in history to be an archaeologist,” she says.