Using lidar to uncover archaeological landscapes at Angkor

Article: “Uncovering archaeological landscapes at Angkor using lidar”

Damian H. Evans, Roland J. Fletcher, Christophe Pottier, et al

www.pnas.org/cgi/doi/10.1073/pnas.1306539110

While there has been a long history of archaeological investigation of the structures associated with Angkorian civilization, our understanding of the complex system of civic-ceremonial centers, high-density and low-density urban environments, water management systems, and agricultural space has been incomplete and flawed.  One major reason for this is the thick forest that covers the structural remains.  With the development of lidar, the vegetation is, in effect, removed.  This article describes the use of lidar and ArcGIS processing in Cambodia at the site of the medieval Khmer Empire complex of Angkor.

In 2012, a block of territory covering the forested area within most of the Angkor World Heritage site was scanned to map variations in surface topography in both horizontal and vertical planes.  Locations were recorded using GPS.  Data points were processed using a method specifically developed for archaeological surveys in forest environments.  These were processed into DEMs, hillshade models, and local relief models using ArcGIS, and there were then analyzed and interpreted.   Archaeologists then went out into the field to verify the results using data loaded into portable GIS units. 

To illustrate a portion of the analytical part of this process, the article includes two visual examples.  In Fig. 2 two layers are presented, one a digital orthophoto mosaic showing elevation from the lidar digital surface model, and a layer showing the extruded lidar DTM with 2x vertical exaggeration.  Modern roads and canals are also shown.  In Fig. 3, a lidar DTM, a conventional satellite image showing limited features due to the vegetation, and a map of previously documented archaeological features shows the extent to which lidar has revealed new structures.

This use of GIS processing, with the precision of GPS location technology and the ability of lidar to strip away vegetation and reveal even slight variations in the ground surface, has transformed our knowledge of the extent and layout of the Angkorian complex.  First, it revealed that the urban center was at least 35 square km in area, rather than the 9 square km that was previously thought to be its extent.  It also revealed that the urban landscapes are not confined within the enclosed or walled city, but extend far beyond.  Much of the extended urban landscape features conform to what was known to be a predictably aligned formation in a grid pattern, but lidar revealed some new types of urban features which have no apparent agricultural, occupational, or hydrological function.

As a result of this study, crucial areas have now been mapped and analyzed, and our understanding of the layout and extent of the Khmer civilization at Angkor, and the factors associated in its decline, has greatly increased.