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June 2004 |
Geomatics in archaeology High tech for old digs
Remote sensing
Remote sensing has long been of interest to archaeologists, because the first problem we face is finding archaeological sites. It is important to remember that archaeologists routinely work in remote areas of the world with few roads or easy access, and that many maps are old and inaccurate. The development of 'aerial archaeology' dates back to the days of balloons and kite cameras, back to before the invention of the aircraft. An aerial balloon photographed the Stonehenge magalith in England in 1906. Once aircraft began taking pictures, archaeologists were among the first to take advantage of this new perspective to locate and map archaeological sites. In the First World War, archaeologists on both sides in Europe and the Middle East that were flying as pilots or aerial observers saw many round, square and linear patterns that they suspected were the vague expressions of ancient landscapes (Dual 1969). After the war they revisited these sites and realized that they were correct, and the new field of 'aerial archaeology' was born. Charles Lindbergh became an early proponent of aerial archaeology and recorded many sites from the air in Central America and the Southwestern U.S. in the 1920's. Aerial photo acquisition and analysis quickly became a standard tool for archaeologists, and these were used to locate and map sites and their environmental locations, and to determine access into remote areas. Thousands of archaeological sites around the world have been discovered in this way.
Archaeologists also quickly adopted close range photogrammetry to record and analyze structures, rock drawings, and other features. Close range vertical photography systems routinely record excavations as they proceed. This is all very important, because archaeology is a destructive process. As we excavate a site we destroy each layer as we proceed, so creating accurate records is a vital part of what we do. The traditional and very slow method of hand drawing each feature and layer has been largely replaced by film (and now digital) recording systems that also allow digital image analysis and entry of the data into a GIS environment.
As soon as satellite imagery became available, archaeologists again quickly investigated the potential of this new capability for our particular needs. Early satellite imagery was limited by low spatial resolution, but the ability to see large areas from space was very useful. This is particularly true as large areas of the world (much of the Middle East, Greece, and Turkey for example) still do not allow aerial photography. As new satellite systems were developed, archaeologists quickly found new uses for them (Madry and Crumley, 1990), although the high cost of satellite imagery (and all advanced technologies) is a real problem for archaeologists. In areas of the world with dense vegetation cover, such as Central America and parts of Asia, RADAR systems provide powerful tools for locating archaeological sites. Several archaeologists had success with the NASA SIR-C/X-SAR system at diverse locations such as Angkor Wat and in locating segments of the Silk Road in Asia, as shown in the images below. In extremely dry areas, RADAR systems can actually penetrate several meters into dry sand, and significant ancient sites were located in the Sahara desert using the SIR-C/X-SAR system (McHough et.al 1988). New LIDAR systems are now providing archaeologists with ultra detailed elevation maps that actually show us the subtle expressions of ancient landscapes such as sunken roadways and buried structures.
SIR-C/X-SAR image of Angkor taken from Space Shuttle "Endeavour" during the Sep/Oct 1994 mission showing an area approximately 55 km by 85 km. North is towards the top right corner. The dark area at the bottom left is a lake, the Tonle Sap. The bright square is the walled central enclosure of Angkor Thom and the enclosed rectangle just below it is Angkor Wat. The two great barays or 'reservoirs' can been seen on either side of Angkor Thom. The Preah Khan baray is just off the NW corner of Angkor Thom and the Indratataki baray is visible as a long, narrow rectangle in the middle bottom of the image just north of the Tonle Sap. Numerous features can be observed well to the north of Angkor Thom especially in the light purple area. The pale blue-green region at the top of the image is the Kulen hills.
Image courtesy of NASA Jet Propulsion Laboratory (Pasadena), the World Monuments Fund (New York) and the Royal Angkor Foundation (Budapest).
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