In Cambodia epic urban landscapes unearthed by airborne archaeologists have rocked the archaeological world and we had an exclusive on the story for The Guardian. A recent airborne laser scanning survey of remote temples has produced remarkable outcomes, rewriting the history books.
In Cambodia Epic Urban Landscapes Unearthed by Airborne Archaeologists
Lara and I have been covering huge archaeological news here in Cambodia on epic urban landscapes unearthed by a 2015 airborne laser scanning survey, which broke yesterday in a story we had published in The Guardian. It’s a story that both confirms theories about the development of Khmer society, agriculture and urbanism, and debunks some myths, and it’s a story that we’ve been close to and with which we have a history.
The first airborne laser scanning survey in Cambodia
When we moved to Siem Reap in mid 2013, one of the first stories we covered was the release of a peer-reviewed report on the first groundbreaking Cambodian airborne laser scanning (ALS) survey, completed in 2012 by the architect of the Cambodian Archaeological Lidar Initiative (CALI), Australian archaeologist Damian Evans, and local teams of archaeologists.
The 2012 survey confirmed that the ruins of the 9th century city of Mahendraparvata, long suspected of being hidden beneath the jungle floor of the densely forested mountain plateau, Phnom Kulen, did in fact exist. Dubbed “the lost city” by media (including our own editors at The Guardian) it was never really ‘lost’, rather its scale was partially revealed.
That 2012 survey also revealed that there was an incredibly dense and complex urban landscape around the temple-city of Angkor Wat and other temple sites such as Beng Mealea and Koh Ker, suggesting these were satellite cities connected to Angkor.
The latest 2015 airborne laser scanning survey
In the recent 2015 airborne laser scanning survey, again headed by Damian Evans, now a Research Fellow at the École française d’Extrême-Orient (EFEO) in Siem Reap, the team gathered data on a far greater area covering a number of remote archaeological sites in northern and central Cambodia, unearthing epic urban landscapes in the process.
This time at Mount Kulen the survey revealed that the 9th century city of Mahendraparvata had covered a continuous area of a colossal 40–50km², a significant discovery by any measure.
There were an array of other impressive findings from across Northern Cambodia with epic urban landscapes identified at other far-flung temple sites, particularly sprawling Preah Khan of Kompong Svay.
There were also fascinating discoveries at the pre-Angkorian temple city of Sambor Prei Kuk, the garrison-city of Banteay Chhmar, and the post-Angkorian capitals of Longvek and Oudong not far from Phnom Penh.
Why this is groundbreaking and gamechanging
Why these airborne laser scanning surveys are proving so groundbreaking is that the old tropical-forest civilisations in monsoonal Asia only used masonry for temples, bridges and features related to water management, such as moats.
Domestic buildings and other structures in urban and agricultural areas were made of earth and wood.
Easily perishable in the harsh climate, the structures disappeared centuries before modern-day archaeologists arrived to hack their way through jungles to see stone temples, inscriptions and works of sculpture and art as proof of these great civilisations.
Until recent years, archaeology was a lucky dip of guesswork and shards of pottery, and, even if a dig was successful, the relics found did not contribute to assessing the extent of the urban landscape and its settlements and structures.
Enter airborne laser scanning (ALS), a cutting-edge technology that is game-changing.
How Airborne Laser Scanning Surveys Work
In the 2015 survey, a Leica airborne laser scanner and a 60 megapixel Leica camera were mounted to the helicopter’s right skid pad. The altitude and positional data of the helicopter were measured by other instruments mounted on the helicopter.
The aircraft flew at a pre-determined altitude (generally 800–1000m) and a pre-determined path at a pre-determined airspeed (∼80 knots), all the while hitting the terrain with over 16 laser beams per square metre. The time the laser pulse takes to return to the sensor is a function of distance and determines the elevation of each individual data point.
The tens of billions of data points collected during the 90 hours of flights are then calibrated, and corrected for aircraft position and aircraft attitude, based on GPS (Global Positioning System) data and IMU (Inertial Measurement Unit) data. This ensures that the data is as accurate as possible.
The most complex part of the process comes next — sorting the ground from the trees. At this stage there is a 3D point cloud — a 3D model of all the information captured during the flights.
To get an accurate terrain map, the essential principle is that any sudden and radical changes in ground height are probably trees — but this is calibrated depending on the terrain that has been flown over. For instance, the parameters in processing this data would be different for outback bush country in Australia verses a pine forest in Cyprus. The technicians processing the data fine tune this to get the most accurate results.
After this data processing (this survey, the largest ever undertaken, took several months of number-crunching), the final 3D point cloud is given to the archaeologists who can then feed this information into a GIS (geographic information system). This allows the archaeologists to create a ‘true’ map of the terrain. In Cambodia, this unearthed epic urban landscapes, such as the one on Phnom Kulen — a truly striking discovery that had been impossible to grasp the scope of through ground work alone.
The challenge of collecting this data in Cambodia is complicated by the weather, as finding the perfect time of year to do these flights is tricky. It is best to do it at the end of the dry season when there is the least amount of leaves on the trees to help with accurate data acquisition.
While this may coincide with the local farmers doing their annual burn-off, the reduced ground cover helps with collecting data. On the downside, smoke and haze can be a problem which is why the archaeologists pray that the first of the monsoon rains come early, but are not heavy enough to hamper flight operations. It’s a gamble when the survey is costing over one million Euros.
Confident after the ground-breaking results of the first survey in 2012, which covered 370km², Dr Evans undertook a far more ambitious 2015 survey covering 1,910km². As the results of the 2012 survey are still being analysed and interpreted, Dr Evans sees this 2015 survey informing research papers and further field digs for many years to come.
Discoveries that will rewrite the history books
While the discoveries about the city of Mahendraparvata were astonishing enough on their own, findings at several other sites are significant. At the temple complex of Sambor Prei Kuk, a capital of the pre-Angkor period (8th-9th centuries), a complex water management system, once thought to be an Angkor-period invention, has been discovered.
The area around Sambor Prei Kuk is also far more densely urbanised than previously thought and the form of an ancient highway has also been identified, connecting Preah Khan and Sambor Prei Kuk. This runs counter to the theory that ‘all roads lead to Angkor’ and suggests a provincial loop or ring-road that may alter how archaeologists envisage transportation and communication channels between these once strong urban centres.
At two significant other provincial centres of the Khmer Empire, Banteay Chhmar (known for its spectacular walls covered in vivid bas-reliefs) and Preah Khan of Kompong Svay (best known for being heavily looted), the findings are disparate.
At Preah Khan of Kompong Svay, the discoveries indicate that there was a distinct urban layout within the central moat, contradicting a recent ground survey that concluded that this area was only thinly inhabited.
ALS data for Banteay Chhmar, on the other hand, has showed little evidence of an urban grid, reducing its likelihood that it was a planned city and possibly suggesting that it was a planned urban centre that was not completed.
Perhaps the most intriguing findings are related to the theory that the Angkorian empire met a catastrophic demise in the 15th century, often attributed to a Siamese invasion. If this theory held true, this would suggest a mass exodus of the urban population of Angkor to the post-Angkorian capitals of Longvek or Oudong, both close to Phnom Penh.
The data collected, however, indicates that these centres were not extensive enough to inhabit a diaspora of those massive numbers of exiles from Angkor. Such findings upend all current theories and will eventually lead to a rewriting of the history books.
“Putting People Back into the Picture”
While currently prohibitively expensive, these airborne laser scanning surveys may one day become common hi-tech techniques in the arsenal of archaeologists. An airborne laser scanner can indeed be attached to a drone for short surveys.
Until then, the romantic view of archaeologists making major discoveries by digging a little patch of ground while sweltering under a burning sun will persist. Yet it’s ironic that Damian Evans and his team at the EFEO are in an air-conditioned office stacked full of humming computers, making discoveries that will end up telling us far more about the bigger picture of how the urban population actually lived.
As Dr Evans said in a video interview we did with him recently in the leafy grounds of Wat Enkosei in Siem Reap, “One of the great values of this data is that it puts people back into the picture. Instead of these cold grey sandstone monuments, what you all of a sudden have is an understanding of them as lived-in spaces, as places where people were born and lived and died and built cities.”
Click through to watch our video interview with Dr Damian Evans on the 2015 Cambodian lidar survey, how lidar works, and the most significant discoveries of the Cambodia airborne laser scanning surveys of both 2012 and 2015.
Note: while it should be obvious this content is original and is copyright, Lara has already had her Guardian story, including interviews we conducted here in Siem Reap on audio and video-tape, plagiarised. If you’re an editor looking for a story, email us.