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The use of airborne thermal imaging as a investigate technique
In archaeological reconnaissance; a report from the Thomson-Ziegler Andean Research Expedition 2003.

By Gary Ziegler

During May 2003 the expedition chartered a high wing Cessna 208 aircraft based at the airport in Cusco, Peru for aerial reconnaissance. Using a Raytheon Palm IR-250 hand held camera; we over flew selected areas near the Inca sites of Machu Picchu and Choquequirao in the Vilcabamba Range west of Cusco. This is a rugged mountainous zone with deep valleys and steeply sloped peaks and ridges climbing up to an altitude of more than twenty thousand feet. Most slopes and ridges are covered with dense cloud forest vegetation up to around 12,000 feet.

Our objective was to test the application of thermal infra-red scanning technology as a means of identifying man made features; trails, stone walls, raised platforms associated with Inca activity in the region. Limited by funds and time we selected several areas that we knew well from previous ground reconnaissance. We chose the IR 250 camera based upon the recommendation of Tom Severs of NASA and research of the available cameras most likely to produce results in this application. This camera has a spectral range of 7.5 to 13 microns.
It filters out the reflect radiation of the sun, and only detects radiation in the far infrared where solar heated stonewalls should be warmer than their surroundings. From a distance of1000 feet the IR 250 can resolve objects on the ground that are 6 inches across and can detect a temperature difference of as small as 0.1 degrees.

In theory, the camera should be able to pick out geometric shapes such as straight and curved lines; rectangular structures and other man made features differing from natural rock outcrops and erosional debris. An unknown was if these features would be revealed when covered with up to several meters of overhead vegetation. When so covered, stone should radiate a cooler thermal signature than warmer living vegetation. If exposed directly to sunlight for some time during the day, stone would appear warmer or darker in the camera's black and white monitor and upon captured images.

The choice of flying days was limited to our schedules and availability of the aircraft. Weather is always an uncertainty with frequent cloud cover in this part of the Andes. We determined that mid day would be the optimum time for scanning if the day was clear and allowed solar warming. However, the prevailing weather pattern for a nice day is clear in the early morning with increasing clouds as the day progresses. Visibility for navigation and safe flying is the paramount factor, which dictates an early morning flight. As a result, our flights were completed before mid morning.

The Cessna 208 has a large rear window that can be opened during flight. The camera was hand held through the open window. Actual images are viewed through the camera viewfinder during operation while a feed cable transports images to a separate digital video camcorder. We used a Small Canon NTSC ZR 60 for data storage.

Data was later transferred by connecting the camcorder to an Apple G-4 computer with video editing capability via a fire wire cable. Individual frames were selected, saved and enhanced in Adobe PhotoShop for viewing and study.

Approximately 60 minutes of images were gathered during two morning flights of several hours each. Our primary objective was a survey of the area directly west of Machu Picchu where we later located the Inca complex of Llactapata during field investigation. On the first flight the region was clouded in so we choice an alternate area of interest some 25 miles to the west then in the clear. We flew around ridges and into the canyons near the sites of Choquequirao and Cota Coca.

During the final flight we concentrated on the Llactapata region and areas of interest nearby. Clouds built quickly limiting scanning opportunity and made flying quite dangerous. We returned to Cusco shortly before cloud cover obscured the entire region.

Although clouds and other factors hindered scanning of the primary objective, enough useful data was gathered to indicate that the technique could be used with some success. The probable route of a abandoned Inca road was identified and several likely sites were located that offer opportunity for later field investigation

It is undetermined if features can be located beneath thick vegetation. The technique helps to identify features such as covered pathways or trails, which may present vegetation differing from the surrounding terrain. Man-made features are readily defined and appear easier to distinguish from natural terrain features than with normal photography. Distance from the target area is important. Scanning sensitivity is greatly reduced as distance increases. Large features were visible at a range of 1000 feet or less. More than this distance would probably be unproductive for all but the largest features.

The matching of images with location proves to be a problem. A system of coordination between frame numbers and a topo map location is necessary. Accompanying still camera photos would be helpful. A team of at least three is needed to operate the equipment and one other to serve as navigator and coordinator with the flight crew. Internal communication via headsets is required.

The use of a helicopter is suggested. A fixed wing aircraft is difficult and dangerous to fly close in to hillsides and narrow valleys. Speed must be maintained and irritate movement at high speed is unavoidable. Navigation between mountains and through clouds is difficult. Previous familiarity of the region and the ability to identify location from the air is imperative.

Our study demonstrates that the use of an airborne thermal imaging scanner can be useful in remote archaeological reconnaissance but that other techniques and approaches need to developed.


Figure1 Choquequirao

Figure 2
Inca trail from Machu Picchu toward Llactapata revealed

Figure 3
Inca trail remains revealed

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Gary Ziegler is an archaeologist, mountaineer and explorer who has spend a lifetime studying the Incas and remote regions of Peru. He is co-owner of Adventure Specialists, a Colorado ranch based adventure tour operation that runs educational treks, horse trips and research expeditions in Colorado, Peru and Mexico's Copper Canyon. His expeditions have located the important Inca sites of Corihuayrachina, Cota Cota and Llactapata. He is a Fellow of The Royal Geographical Society, The Explorers Club and a sometime lecturer at Colorado College. He can be contacted at: