BioEd Online

A comprehensive map of the ancient Cambodian metropolis of Angkor has been produced from a radar survey, supporting the notion that this sprawling city was irrigated to produce food for a substantial population.

Founded in the ninth century AD and abandoned 700 years later, the city of Angkor was carved out of the jungle and was home to the massive medieval temple of Angkor Wat. Archaeologists have long wondered whether the enormous reservoirs and kilometres of canals and ditches of the 'greater Angkor' surrounding the famous temple were used for agriculture to support what would have been the world's largest pre-industrial city, or whether they served some other purpose for a smaller population.

The radar survey shows a bustling centre connected to smaller suburbs, with an extensive and regular network of fields and canals. "The scale of the landscape manipulation is unparalleled," says Damian Evans, an archaeologist at the University of Sydney in Australia, who led the mapping project. This supports the idea that Angkor's residents channelled the Siem Reap river to feed a population that may have exceeded a million.

The 1,000-square-kilometre settlement resembles modern cities such as London and New York, says Evans.

Their map reveals previously unseen details of this sprawling city, including some 79 'linear features' that could be remnants of old canals or roadways, 94 local temples, and two giant earthen mounds of unknown use.

Urban sprawl

Angkor and its temples have garnered the attention of many archaeologists, some of whom have attempted to map the giant site. In the 1950s, for example, French archaeologist Bernard Philippe Groslier led an aerial and land-based mapping project. But this was left unfinished due to funding cuts and political turmoil in Cambodia. More recent mapping efforts have also been incomplete or lacking detail.

Groslier hypothesized that a massive water network in the city propelled the growth of Angkor as more and more irrigation fed more and more people. He added that overuse of the area's resources probably led to the city's collapse.

This theory was questioned by archaeologists in the 1980s, who said there was little proof that the city supported such a massive population. Instead, the canals could have served as a means of transportation for a smaller population during the rainy season, they proposed.

But the new radar map paints a picture of a landscape moulded by a large resident population, says Evans. Rice cultivation sparked the expansion from Angkor's centre and the hydrological network to support it. "Angkor's economy revolved around rice," he says.

Reported in the Proceedings of the National Academy of Sciences this week¹, the new plan was made from radar measurements taken by a NASA aircraft as it flew over Angkor in 2000. With the help of the agency's Jet Propulsion Laboratory in Pasadena, California, Evans's team converted the radar measurements into a topographical map.

Water fall

Left unanswered is the role played by water in Angkor's eventual fall. A previous excavation turned up silt in its canals, suggesting that the water system failed at some point. More fieldwork is needed to determine whether an environmental collapse led to the city's abandonment, says Evans, who will return to Angkor in early 2008.

The map is "a milestone in our understanding of Angkor", says Charles Higham, an archaeologist at the University of Otago in New Zealand. But Higham adds he is still unconvinced that so many people lived in the area, and says more excavation work is needed to convince the sceptics.

With the map, archaeologists have a new tool to focus future excavations in greater Angkor. But looters could undermine their efforts, says Evans. The central city and its great temples are protected as a UNESCO World Heritage site and patrolled by police, but most of the sprawling suburbs fall outside that zone.

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