At its apogee in the 13th century CE, the ancient city of Angkor was home to approximately 700,000 to 900,000 inhabitants, according to a new study built on three decades of research that places Angkor on the map as one of the world's largest medieval cities.

The authors of the study, published Friday in Science Advances, combined lidar assessments, archaeological excavation data, radiocarbon dating and machine-learning algorithms to create maps that modeled Angkor's development and its population over time — an innovative prehistoric demographic model of one of the most important archaeological sites in Southeast Asia. Lidar, short for Light Detection and Ranging, is a sensing method that sends pulses of laser light to determine the presence, shape and distance of objects.

The ruins of Angkor — a UNESCO World Heritage Site largely left intact — are located in Cambodia. Angkor was the capital of the Khmer Empire from the 9th century to the 15th century CE. Angkor Wat, the famous religious temple built in the 12th century CE that served as the state temple for the empire, remains the largest religious complex in the world, based on land area. 

The biggest challenge in estimating Angkor's population size and density is that although religious architecture made of stone and brick is in an excellent state of preservation, nonreligious architecture such as houses in the region were made almost entirely of organic materials, including wood and thatch, that decayed centuries ago.

Alison Carter, a co-author of the paper and an assistant professor at the University of Oregon, told The Academic Times that she loves teaching about Angkor in a world archaeology class she conducts every year.

"I get a kick out of kind of introducing it to people for the first time because it's so spectacular," she said. "I always just, like, love showing pictures. The first time I saw a picture of Angkor Wat, I was like, 'What? I've never seen anything like this before. I have to go there.' And here I am, like, 20 years later, doing research there. The initial kind of amazement really hooked me."

Angkor's demographic growth is the subject of "persistent speculation and controversy," according to Carter and her colleagues, with previous estimates of population size as large as 1.9 million people in the Angkor region. However, the present study's model falls in line with more recent estimates of 750,000 people, based on the carrying capacity of the landscape. 

Because conventional methods for estimating population size and density in an urban area don't work in a place like Angkor, Carter and her colleagues produced the first granular, diachronic model of the Angkor complex by pulling together 30 years of archaeological research, including historical archives and maps, measurements from multiple airborne lidar surveys, fine-grained archaeological excavation data, and machine-learning algorithms.

The researchers used lidar scans to produce a detailed, ground-surface map of Angkor by scanning gaps between the trees that make up dense jungles in the region. Excavation data from the mounds people were living on, as well as ethnographic data, allowed the researchers to estimate population sizes in several areas. Radiocarbon dating and historical art analyses from the stylistic parts of the temples, along with predictive machine-learning algorithms, were also used to pin down dates and model the development and growth of the landscape over time.  

This paleodemographic model revealed that the population density in Angkor's civic-ceremonial center, the location of the royal residence and large stone temples, was comparable to that of other large tropical and subtropical ancient cities, such as Teotihuacan, in Mexico, and Anyang, in China. 

The model shows that at the beginning stages of Angkor's development, population sizes were equal between the countryside and the "downtown" city-center area, according to Carter. Over time, there was a population explosion in the countryside, Carter said, though the population size in the city area stayed consistent. 

Both a top-down and a bottom-up approach to building the countryside seem to be the reasons that the countryside saw more growth than the city area during this time, Carter said, with the rulers and kings of Angkor expanding infrastructure, such as water-management networks. 

Even though people in the countryside were fairly autonomous, making their own decisions about their farming practices and how to use the landscape, having infrastructure from the city seemed beneficial. The kings and rulers also seemed to have established favorable tax policies for people founding temples and "getting the landscape going," Carter said. 

In the late 12th and early 13th centuries, the model shows an explosion of population in the city area — an explosion that researchers think was sparked by policies put in place by King Jayavarman VII, the most famous Angkorian king, who encouraged people to come to the city center. 

These insights elucidate the dynamic nature of agro-urban settlements, and the researchers suggest that future work should look to see whether these dynamics are constant across other ancient societies. Moreover, Carter and her colleagues say future research could be done on the role of migration in Angkor, especially when development of its civic-ceremonial center was at its zenith. 

The researchers say that the paleodemographic model employed in the study can be applied to any ancient civilization, especially those where conventional methods for estimating population size and density aren't available.

Carter also told The Academic Times that she hopes this research causes travelers and researchers to pay more attention to Angkor. 

"I think if this gets people to start paying more attention to Angkor in general," she said, "and putting it on the map as being, like, a major urban center that's aligned with like Rome, London, different Mayan cities, Teotihuacan, that kind of place, then I would be really happy for this to be an entry point for people to start thinking more seriously about Angkor as an important place to study and learn about." 

"It's such a wonderful and fascinating place," Carter added. 

Sources of funding for the study include the Rust Family Foundation, the University of Oregon's Global Oregon Faculty Collaboration Fund, and ACLS-Robert H. N. Ho Family Foundation Program in Buddhist Studies.  

The study "Diachronic modeling of the population within the medieval Greater Angkor Region settlement complex," published May 7 in Science Advances, was authored by Sarah Klassen, University of British Columbia, University of Oregon and Leiden University; Alison K. Carter, University of Oregon; Damian H. Evans, Pelle Wijker, and Christophe Pottier, École française d'Extrême-Orient; Scott Ortman, University of Colorado Boulder; Miriam T. Stark, University of Hawai'i at Mānoa; Alyssa A. Loyless, Stanford University; Martin Polkinghorne, Flinders University; Piphal Heng, Northern Illinois University; Michael Hill, independent scholar; Jonathan Niles-Weed, New York University; Gary P. Marriner, Casey & Lowe Archaeology and Heritage; and Roland J. Fletcher, University of Sydney.