Revealing Secrets of the Angkor Civilization Using Airborne LiDAR Technology

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In the first ever project of its kind in South East Asia, PT McElhanney Indonesia acquired LiDAR and digital aerial photography in order to map Angkor Wat and other ancient features in Cambodia to a degree of accuracy that has never been seen before by Cambodian archaeologists.

Angkor Wat is one of the world’s most recognizable temples. Designated a UNESCO world heritage site in 1992, this temple was built by the ancient Angkorian civilization in the 12th century by King Suryavarman II. Angkor Wat is the center of a once massive and powerful nation suspected of being the largest in the world at the time with up to one million citizens. Supporting such a population would require a large infrastructure of water works during the arid Cambodian dry season. Previously, archaeologists have used radar and remote sensing data to try to make sense of the greater Angkorian civilization. It is suspected that a sophisticated water infrastructure consisting of reservoirs, canals and damns captured the water flow from the highlands and was distributed throughout the rice paddies of the lowlands.

Archaeologists trying to make sense of this civilization have many challenges while trying to map out these features. The remote areas in the hills are hampered by jungle and are still laden with landmines from the Khmer Rouge era. Other areas not as remote have been built up and over generations altered by citizens cultivating their land which makes ancient feature identification from the ground virtually impossible. Over the centuries, the historical records of this civilization have been lost due to conquering armies ransacking archives and the harsh tropical climate on literature that was once recorded on palm leaves. Remote sensing is the perfect technology for these challenging conditions.

Using a helicopter-mounted Leica ALS60 LiDAR scanner and Leica medium format digital aerial camera, PT McElhanney Indonesia proposed to fly these areas in order to pinpoint and model key features for the archaeologists. This would enable them to isolate the subtle topographical changes of water infrastructure and other urban civilization planning in order to identify areas of interest and then organize landmine clearing crews to make it safe to inspect on the ground.

LiDAR Consortium
A consortium was necessary to put a project of this scale together. PT McElhanney worked closely with Professor Roland Fletcher and Dr. Damian Evans from the University of Sydney’s archaeology department to organize this group. Areas of interest were gathered and interested parties were brought together by Dr. Evans to create the Khmer Archaeology Consortium (KALC). The following parties were directly involved with this project:
Authority for the Protection and Management of Angkor and the Region of Siem Reap (APSARA) [Cambodia]
cole Franaise d’Extrme Orient (EFEO), Siem Reap Centre [France]
University of Sydney, Robert Christie Research Centre (USYD) [Australia]
Socit Concessionnaire des Aroports (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]

A website has been designed by the consortium to help promote the survey and share results as the project progresses. The project scale will mean that new developments will continue to evolve for years to come.

Project Preparation
Before mobilizing the equipment to site, the project scope had to be understood by all parties. A ground reconnaissance trip was made and time spent with the archaeologists to understand what information they were hoping to reveal from the data. Another important aspect of this trip was to assess the terrain and vegetation on site to maximize the LiDAR’s potential.

PT McElhanney spent a couple days travelling to various sites with the archaeologists to understand the challenges that they faced. In the lowlands and built up areas, site inspections of excavation sites were conducted. In these areas the archaeologists were hoping to get a better understanding of where the "occupation mounds" were located. These are subtle rises in topography that usually indicate some extent of habitation above the rice fields or drainage areas. Understanding that the inhabitants, including the King, lived in wooden structures made it clear that there was no possibility of residential building foundations being easily identified. This was critical in designing a LiDAR dataset that would identify these subtle topographical changes. Only temples were made of stone so being able to identify possible stone debris was also very important. By having the combination of high resolution aerial photography and LiDAR, this was going to help locate and identify these high interest lowland areas.

The vegetated areas posed a greater challenge. Even though the requirements for subtle terrain change were the same as the other areas, the dense Cambodian forests coupled with the random locations of the landmines meant that the archaeologists could not freely investigate areas of interest. Instead, the very time consuming and costly requirement of sending in landmine clearing crews to clear these areas was a prelude to all investigation. Identifying these areas with satellite imagery and radar data under the forest canopy was virtually impossible so LiDAR had been proposed to give them the best Digital Terrain Model (DTM) available so they could concentrate future inspections to specific locations. The high accuracy LiDAR DTM could also aid in modelling features that previously were incorrectly identified such as roads which were in fact reservoir walls.

Due to the danger of landmines throughout Cambodia and the relatively little exploration that has been conducted in remote areas, it is suspected there are still many undiscovered temples to this day. LiDAR was also identified as the technology to help identify additional temples that may inhabit the project areas of interest.

Project Execution
Once the very exclusive flight permits were issued from the authorities in Cambodia, PT McElhanney proceeded with the mobilization plan and mounting their equipment on Helistar Cambodia’s Ecureuil AS350 helicopter in Phnom Penh.

The total project had three main areas of interest around Siem Reap covering an area of 270 square kilometers. Due to the high level of accuracy required, it was proposed that two main GPS base stations would be used for acquisition. The helicopter was also ideal since one particular area was approximately 100 kilometers from the nearest airport so on-site refuelling would be required during the acquisition of that block.

In the forested areas, acquisition was flown using a cross hatched pattern to maximize opportunity for LiDAR penetration to the forest floor. These areas were acquired with a LiDAR point density of up to 16 points per square meter which was essential for the modelling required beneath the forest canopy. Over temples, this approach also maximized the 3D modelling ability of the LiDAR scanner. Full waveform LiDAR was also collected over the areas of interest to ensure the best DTM would be available in the heavy grasses and vegetation present on some of the sites.

Digital aerial photography was simultaneously collected with the LiDAR data. Even though the aerial photography was a secondary product to the LiDAR, this was collected to a stereo image specification and had complete overlap of all the LiDAR for downstream products and viewing. This high resolution imagery would be invaluable when completing the LiDAR modelling.

All of this data together and the densities collected amounted to a very large dataset being acquired on a daily basis. Data management these days is one of the biggest challenges during data acquisition even with current, state-of-the-art computing power. A daily challenge was to have all of this data backed up and the drives ready for acquisition the following morning.

Aerial acquisition challenges were also present while operating during the arid Cambodian summer. Afternoon temperatures often exceeded 45C+ which pushed the operating temperatures of the equipment to the limit. Summer slash burn by the local farmers caused challenges to ensure the highest quality of photography could be captured. All of this was overcome without major delays and the project remained on schedule.

LiDAR in Archaeology
Preliminary analysis is already revealing amazing new discoveries for the archaeologists. According to Dr. Evans, the LiDAR survey has produced "a lifetime" of data in a short period of time. LiDAR’s ability to penetrate dense vegetation has meant that the temple complexes are able to be seen without vegetation for the first time most likely since the habitation period. Subtle topographical changes have traced out road networks, occupation mounds and other urban planning signs that were previously undetectable even from the ground. Archaeologists are already speculating that the data will provide a completely new insight in the Angkor civilization and revolutionize the history of the Khmer Empire.

The project is already garnering attention in the region. It has been nominated and declared winner of Asia Geospatial’s Award of Excellence Archaeological Application which was handed out in September 2012. Further nominations and regional recognition will hopefully follow and revolutionize the technology’s use in archaeology throughout this region and the world

Chris Cromarty is the LiDAR Manager of PT McElhanney Indonesia, a division of McElhanney Consulting Services Ltd., based in Vancouver, Canada. Chris has an extensive background in LiDAR and remote sensing with over 15 years’ experience in mapping projects around the world and 5 years of operations in South East Asia.

A 1.993Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE