Pulling artifacts from the jungle using Full Waveform LiDAR

Author: Penny Boviatsou

The greatness of Khmer Empire as well as its importance as part of the Cambodian cultural heritage is just beginning to unfold. The most extensive airborne laser scanning campaign ever conducted on an archaeological project revealed a massive complex of ancient cities buried in the Cambodian jungle.

LiDAR is proving to be a catalyst for a better understanding of our history. The innovative technology is transforming archaeology and is fast becoming a powerful tool for archaeologists; the lasers can penetrate overlying vegetation and cover swaths of ground extremely faster than they could be studied on foot.

The Angkor complex is one of the most important archaeological sites in Siem Reap in Southeast Asia and the Angkor Wat one of the world’s most recognisable temples. In the late 12th and early 13th centuries the city was at its peak with historians describing an urban centre extending over nearly 1036 square km. Angkor was the centre of a once massive and powerful nation believed to be the largest in the world at the time, with up to 1 million inhabitants.

Hidden but not forgotten

Nothing ignites a researcher’s imagination like the prospect of a lost metropolis. Archaeologists in Cambodia after numerous years of research have recently announced the discovery of multiple medieval cities hidden beneath the tropical forest floor near Angkor and other archaeological sites across the Kingdom of Cambodia. The colossal discovery challenges the theories that the Angkorian people eventually abandoned the cities for ones in the South as it was until recently believed. The archaeologists were able to discover the ruins of the lost civilisation thanks to airborne laser scanning technology:

• The Khmer Archaeology LiDAR Consortium (KALC) in April 2012 and the
• Cambodia Archaeology LiDAR Initiative (CALI) in April 2015 decided to widen the perspective beyond conventional methods to Khmer history and archaeology.

Both projects were designed by Francisco Gonçalves and Chris Cromarty from PT Map Tiga Internasional (PTMI - formerly PT. McElhanney Indonesia), a Jakarta-based company providing aerial mapping and surveying expertise.

The premise for the projects was that LiDAR could provide key information hiding under tree canopy, needed to fully assess the extent and impact of the Khmer civilisation in Southeast Asia, particularly in Cambodia. To accomplish the goals, several meetings tool place between Francisco Goncalves, Professor Roland Fletcher and Dr. Damian Evans. These allowed PTMI to understand the landscape and vegetation challenges, site visits were also planned prior to acquisition.

PTMI acquired Leica Geosystems’s LiDAR and digital aerial photography sensors to provide the researchers with high resolution aerial imagery as well as high density ground LiDAR points to the archaeologists while scanning the Cambodian jungle.

The first findings came in 2012 when the company worked closely with Dr. Damian Evans and the Consortium’s associates. By flying a helicopter mounted with a POD carrying, a Leica LiDAR scanner equipped with waveform data capture and a Leica digital aerial camera, PTMI gathered data that confirmed the existence of Mahedraparvata, an ancient temple city whose existence had been known for decades.

The recently analysed data, captured in 2015 during the most extensive airborne study ever undertaken by an archaeological project, uncovered the total scale of the Khmer Empire’s urban extent and found the temple complexes to be extremely greater than was previously believed.

“LiDAR generated maps revealed multiple medieval cities hidden on the forest floor and upended what we knew about the lived-in spaces around Angkor and other temple complexes across Cambodia,” said Dr Damian Evans. “The vegetation was obscuring these monumental sites. The LiDAR technology allowed us to see through the vegetation.”

Unveiling the secrets of an empire

In 2015, PTMI covered 1600 square km with the goal to deliver the Cambodia Archaeology LiDAR Initiative, the finest and most complete data coverage of their areas of interest, and provide an opening for future archaeology LiDAR investigation. At the planning stages of the project, PTMI spent approximately a year discussing approaches to the LiDAR and Digital Air Photo (DAP) acquisition.

Francisco Gonçalves and Chris Chromarty walked areas planned for LiDAR and DAP acquisition, and studied the terrain and vegetation under tree cover and the type of archaeological features the LiDAR could detect.

“Archaeology fascinates me,” says Francisco Gonçalves, President Director at PTMI. “What secrets are hidden, where our ancestors did built cities and why they abandoned them. Penetrating the Cambodian jungle with the Leica Geosystems technology we were able to acquire LiDAR point data that precisely revealed not only the structure of the land scape but also constructions, roads, canals, structural foundations and agricultural terraces.”

PTMI used Leica Geosystems’s LiDAR mapping systems to scan the Cambodian jungle. The company collected data with LiDAR technology that penetrated the dense Cambodian forests and thick vegetation, gathering information about the soil’s surface and terrain elevation that enabled archaeologists to find traces of extensive networks around Angkor. Digital aerial photography was simultaneously collected with the LiDAR data.

"Agricultural fields and other structures have remained hidden by growing trees for centuries, but LiDAR technology is changing the way we view the world," said Professor Roland Fletcher. "Cases of recently deforested lands challenge the theories that ancient forests are 'pristine ecosystems', people shaped the landscapes where forests are located for thousands of years."

All LiDAR data was acquired using a POD mounted in an AS350 B2 helicopter (HeliStar) based in Cambodia and then adjusted and processed by PTMI using processing workstations equipped with DataPort removable drive carriers, allowing easy movement of the large data files from the aircraft to the processing centre. PTMI planned the data acquisition, adjusted the air imagery and classified the LiDAR into ground and non-ground points. All the results were delivered to CALI where Evans and his team were able to prepare the archaeological analysis.

LiDAR – a new tool for archaeological prospection

LiDAR’s ability to penetrate dense vegetation has meant that the temple complexes and surrounding areas are able to be seen clearly for the first time since the habitation period. With LiDAR-generated maps, subtle topographical changes have traced out road networks, occupation mounds and other urban planning signs that were previously undetectable even from the ground.

The LiDAR-made maps enable the CALI researchers to make more targeted excursions at the site whilst the LiDAR data determine the locations where further research might be useful.

Researchers in the archaeological sphere agree these are the most significant archaeological discoveries in recent years.