Mapping ancient sites using LiDAR tech

Mapping ancient sites using LiDAR tech

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In April 2012, Angkor Wat and other high interest areas in Cambodia were mapped by LiDAR (light detection and ranging) and aerial photography, to provide archaeologists insight into the former Khmer Empire. The Khmer Empire was a major power during 9th -15th centuries in Cambodia. The ruins of its capital lie in Angkor, within present day Siem Reap. Angkor is one of the largest pre-industrial urban complexes in the world. Its most famous temple, Angkor Wat, is a national symbol and depicted on the Cambodian flag. Angkor is one of the most important archaeological sites in Southeast Asia and was designated the UNESCO World Heritage Site in 1992.

Remote sensing activity in Angkor
Since early 1920s, archaeologists have been using aerial photography to help locate and map out the ancient civilisation. Aerial photographs of Angkor were acquired throughout1920s and 1930s. A combined aerial and land-based mapping project was initiated in the 1950s which was left incomplete due to funding limitations and political turmoil. During the 1970s and 1980s, all archaeology initiatives were put on hold and resumed in 1992. A series of radar surveys were undertaken between 1994 and 2000, most significantly AIR-SAR/TOPSAR data acquired by NASA in September 2000.

 

 


Angkor Wat Oblique

In the 21st century, archaeologists started recognising the importance of LiDAR technology. Historical sites such as Caracol and Stonehenge are a few pioneer projects on the world map using such technology. This trend extended into Southeast Asia and specifically for the greater Angkor Wat area.

Use of LiDAR in Southeast Asia
Important historical regions such as Siem Reap and Ayutthaya are believed to have many ancient wonders yet to be discovered. It is estimated that in Siem Reap alone, only a fraction of the ancient infrastructure has been unearthed so far. Use of LiDAR tech could build on previous surveys to reveal more detailed features of the landscape.

Radar imaging in Angkor has provided evidence of new settlements, canals and reservoirs, but the limitation of vegetation penetration makes it less effective than LiDAR, specifically for detecting smaller features and low relief constructions in vegetated areas. While radar produced an elevation value for few metres of landscape, LiDAR generated multiple data points within a single square metre, even penetrating the dense jungle canopy. Its accuracy far exceeds that of other remote sensing technologies, providing a detailed picture of the topography and modifications made by ancient civilisations. Very subtle traces of previous development can still be detected on the surface of the landscape hundreds or even thousands of years later.

To advance the idea of using LiDAR within Southeast Asia, PT McElhanney Indonesia (PTMI), a division of McElhanney Consulting Services Ltd., worked closely with Professor Roland Fletcher and Dr. Damian Evans of the University of Sydney’s archaeology department to get the necessary parties involved to proceed with such a project. Presentations were made and Dr. Evans organised the necessary parties to ensure the funding and interest was in place to conduct this historical project.

The final consortium of interested parties consisted of eight groups who all had specific interest in such data. The Khmer Archaeology LiDAR Consortium (KALC) was headed by Dr. Evans, with the following organisations participating:

  • Authority for the Protection and Management of Angkor and the Region of Siem Reap (APSARA) [Cambodia]
  • École Française d’Extrême Orient (EFEO), Siem Reap Centre [France]
  • University of Sydney, Robert Christie Research Centre (USYD) [Australia]
  • Société Concessionnaire des Aéro-ports (SCA) [France and Cambodia]
  • Hungarian Indochina Company (HUNINCO) [Hungary]
  • Archaeology & Development Foundation (ADF), Phnom Kulen Program [France]
  • Japan-APSARA Safeguarding Angkor ( JASA) [ Japan]
  • World Monuments Fund (WMF) [USA].

APSARA acted as government lead, the EFEO as administrative lead, and the University of Sydney as technical lead. PTMI was the technical advisor. It was responsible for the planning and acquisition of aerial photography and LiDAR data. The survey covered an estimated 270 square kilometres over three major blocks: Angkor and Phnom Kulen in Siem Reap Province and Koh Ker in Preah Vihear Province.

Surveying Angkor
Prior to conducting the aerial survey, a reconnaissance trip was planned in March 2012 for PTMI to spend time on the ground with some of the consortium end users. Several days were spent travelling to different site types understanding the terrain and vegetation types that were present over the areas of interest. Access to some of these areas were beyond 4X4 trails and motorcycles or even by foot was undertaken to try to gain a complete understanding of what the archaeologists were hoping to detect or confirm. Some areas were completely inaccessible by any mode due to the danger of land mines that still plague many areas of Cambodia.

The reconnaissance trip proved to be extremely valuable for designing a survey plan that would try to ensure the best dataset that the technology could provide. It was previously decided that a helicopter would be the best platform for this survey so other logistics such as ground GPS base locations and remote refuelling sites were established.

A few weeks later, the acquisition team returned to Cambodia to install the system in Phnom Penh. At this time, the McElhanney LiDAR pod was installed onto Helistar’s AS350 B2 helicopter. PTMI’s Leica ALS60 laser scanner and RCD105 digital frame camera were mounted into the pod with the remainder of the logging and power units being mounted internally into the helicopter. Ground testing and calibration flight were then conducted in Phnom Penh prior to relocating to Siem Reap, which would be the survey base of operations.

During the next week, daily missions were flown to complete the acquisition phase of the project. The Cambodian summer resulted in hazy mornings followed by extreme heat in the afternoons with temperatures reaching well into 45°C+ which pushed the equipment cooling systems to the limits. Afternoon field burning activities by local farmers could not be avoided so were a constant concern to the acquisition team in collecting the best quality photo possible. One of the project blocks encompassed the Siem Reap International Airport which also resulted in acquisition flights to be scheduled around the regular air traffic.

During acquisition, data quality and coverage were checked with preliminary results being shared with the team as the project progressed. This preliminary data revealed results that exceeded the archaeologists’ expectations of what the survey would provide. Post processing of the data in the office still continues today with findings that are constantly changing what was previously known of the ancient Angkor civilisation.


Angkor Wat Aerial

Archaeology and LiDAR
According to Dr. Evans, LiDAR survey produced “a lifetime” of data—in only one week of acquisition. Data was delivered to consortium members for their own analysis as well as PTMI completing their own analysis to further compare results down the road. LiDAR’s ability to penetrate dense vegetation and to detect subtle elevational differences revealed a vast array of previously undocumented features such as roadways, canals, urban enclosures and temples. This will aid researchers in developing a more detailed map of the sites, one of the primary goals of the Greater Angkor Project.

Preliminary analysis points to important new discoveries that, according to archaeologists in Siem Reap, will revolutionise the history of the Khmer Empire. Of particular note is a new settlement discovered surrounding Beng Mealea, a known temple between Angkor and Koh Ker. The area is obscured by forest cover and the presence of land mines has made ground survey both difficult and dangerous. LiDAR has revealed the area as an intricate, planned settlement with a network of gridded roadways and ponds. It has been impeccably pre-served, offering researchers a first-hand view of a 12th century Khmer city.

The findings in Siem Reap have provided substantial insight into the Khmer civilisation as well as temple complexes worldwide. Many known temple sites may actually be the centres of low density urban areas, currently enshrouded in vegetation or covered by modern settlement. Even in areas studied extensively, LiDAR can detect previously undiscovered features or provide a new level of detail.

In addition to its contributions to archaeological research, the use of LiDAR has significant implications for site and heritage management. LiDAR data can identify areas requiring immediate protection, create baseline and archival documentation, and warn of illegal activities such as excavation, looting, and deforestation. Insight may also be gained into the scale and extents of historical settlements to more accurately assess zone boundaries to ensure these important sites are protected and preserved well into the future.