In 2007, satellite images, radar, and ground surveys revealed Angkor as the world’s largest preindustrial settlement complex

In 2007, a team of archaeologists used satellite imagery, aircraft radar, and ground surveys to create the first complete map of the ancient Cambodian city of Angkor. The findings showed that the city not only includes famous temples but is also the world's largest preindustrial settlement complex, stretching to about 1,000 square kilometers in present-day north-western Cambodia. According to NASA's Earth Observatory, radar mosaics spanning 2,848 square kilometers, constructed between 2000 and 2007, were superimposed on older archaeological maps and recent field surveys. The findings gave researchers a far clearer picture of how the city extended well beyond its temple complexes. Nevertheless, it demonstrates that some of the most important historical discoveries can arise from re-examining old ground with new tools.

A city that hid behind its own temples

For centuries, people who traveled to Angkor Wat believed it was the only building in the area. European traders, who found it at the end of the 16th century, were fascinated by this archaeological find but described the place as completely desolate. The surrounding wilderness devoured everything else. Various excavations and research efforts have focused on understanding this and other Angkorian sites. Researchers focused on stone structures because wooden buildings and other structures were thought to have long since disintegrated. The 2007 discovery changed this perception. When they studied an old theory proposed by the French archaeologist Bernard-Philippe Groslier in 1956, who believed that Angkor was an irrigation city built to regulate a large area prone to unpredictable monsoon rains and housed more than one million inhabitants, they sought to use remote sensing technology to both support his theory and find an archaeological footprint of such a settlement.

Reading a city through radar instead of photographs

Unlike a camera, radar imaging uses reflected signals to create images. This is why it worked better than some earlier surveying methods. The AIRSAR aircraft sent out pulses of energy and recorded the reflected signals that bounced off objects on the ground. Water reflected the signal and appeared as dark areas in the image. Signals bouncing off trees in forested areas made those areas appear brighter and speckled. Straight lines and right angles stood out more clearly in the radar images than they do in the landscape itself. Because of this, radar images could penetrate dense foliage and reveal objects such as roads, reservoirs, and waterways. The National Academy of Sciences reported that every feature on the final map appeared in two data sets or was verified on the ground.

Why the size of an old city is worth knowing today

The authors estimated that the overall settlement, extending well beyond Angkor's walled temple enclosures, covered 900 to 1,000 square kilometers, slightly larger than the land area of present-day New York City, which covers about 784 square kilometers. Angkor was a low-density city, with buildings, farms, and temples scattered throughout the landscape, making it more like a modern suburban area than a medieval European city. This difference was significant when comparing Angkor to other cities: measurements at Tikal, the best-known center of Maya civilization in Central America, showed that Angkor was several times larger in both extent and population. Comparisons with Tikal also showed similarities in density and urbanization. For modern-day suburbs and "checkerboard" developments, this finding can be of interest for comparison and optimization.

The water system that built Angkor, then helped end it

The same infrastructure that helped Angkor thrive may well have hastened its demise. The water system protected the people of Angkor from monsoon deluges and dry-season deficits, allowing them to flourish in one of the world’s largest cities. However, the infrastructure required maintenance, and removing and shifting topsoil to meet the demands of a growing population had environmental consequences, making the canals harder to control and more susceptible to erosion. Researchers found evidence of hasty repairs and breaches in the system, implying that it required more upkeep than it received in its final years.

The 2007 map still guides how Angkor is protected

Angkor is now included on the UNESCO World Heritage List. The UNESCO World Heritage Center notes that the protected area covers more than 400 square kilometers, with over 100 monuments plus hundreds of other man-made objects. The 2007 mapping gave authorities a much clearer idea of what needed protection, including not only the temples at the heart of the tourist destination but also the connected canals, mounds, and roads. The same remote-sensing methods have since been used to identify other abandoned settlements buried by jungle elsewhere, offering new insight into humanity's past.