Remote Sensing

Arenal, Costa Rica

Chaco Canyon, NM

The Petén, Guatemala

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Remote Sensing Tutorial

Archeological Remote Sensing

Now more than ever, archeological research is interdisciplinary: botany, forestry, soil science, hydrology - all of which contribute to a more complete understanding of the earth, climate shifts, and how people adapt to large regions.

As a species, we've been literally blind to the universe around us. If the known electromagnetic spectrum were scaled up to stretch around the Earth's circumference, the human eye would see a portion equal to the diameter of a pencil. Our ability to build detectors that see for us where we can't see, and computers that bring the invisible information back to our eyesight, will ultimately contribute to our survival on Earth and in space.

The spectrum of sunlight reflected by the Earth's surface contains information about the composition of the surface, and it may reveal traces of past human activities, such as agriculture. Since sand, cultivated soil, vegetation, and all kinds of rocks each have distinctive temperatures and emit heat at different rates, sensors can "see" things beyond ordinary vision or cameras. Differences in soil texture are revealed by fractional temperature variations. So it is possible to identify loose soil that had been prehistoric agricultural fields, or was covering buried remains. The Maya causeway was detected through emissions of infrared radiation at a different wavelength from surrounding vegetation. More advanced versions of such multi-spectral scanners (Visible & IR) can detect irrigation ditches filled with sediment because they hold more moisture and thus have a temperature different from other soil. The ground above a buried stone wall, for instance, may be a touch hotter than the surrounding terrain because the stone absorbs more heat. Radar can penetrate darkness, cloud cover, thick jungle canopies, and even the ground.

Remote sensing can be a discovery technique, since the computer can be programmed to look for distinctive "signatures" of energy emitted by a known site or feature in areas where surveys have not been conducted. Such "signatures" serve as recognition features or fingerprints. Such characteristics as elevation, distance from water, distance between sites or cities, corridors, and transportation routes can help to predict the location of potential archeological sites.

Computational techniques used to analyze data.

1. sun-angle correction
2. density slicing
3. band ratioing
4. edge enhancement
5. synthetic color assignment
6. filtering
7. multichannel analysis

Remote Sensing Instruments

Aerial Photography:
Many features which are difficult or impossible to see standing on the ground become very clear when seen from the air. But, black and white photography only records about twenty-two perceptible shades of gray in the visible spectrum. Also, optical sources have certain liabilities, they must operate in daylight, during clear weather, on days with minimal atmospheric haze.

Color Infrared Film (CIR):
Detects longer wavelengths somewhat beyond the red end of the light spectrum. CIR film was initially employed during World War II to differentiate objects that had been artificially camouflaged. Infrared photography has the same problems that conventional photography has, you need light and clear skies. Even so, CIR is sensitive to very slight differences in vegetation. Because buried archeological features can affect how plants grow above them, such features become visible in color infrared photography.

Thermal Infrared Multispectral Scanner (TIMS):
A six channel scanner that measures the thermal radiation given off by the ground, with accuracy to 0.1 degree centigrade. The pixel (picture element) is the square area being sensed, and the size of the pixel is directly proportional to sensor height. For example, pixels from Landsat satellites are about 100 feet (30 m) on a side, and thus have limited archeological applications. However, pixels in TIMS data measure only a few feet on a side and as such can be used for archeological research. TIMS data were used to detect ancient Anasazi roads in Chaco Canyon, NM.

Airborne Oceanographic Lidar (ADI):
A laser device that makes "profiles" of the earth's surface. The laser beam pulses to the ground 400 times per second, striking the surface every three and a half inches, and bounces back to its source. In most cases, the beam bounces off the top of the vegetation cover and off the ground surface; the difference between the two give information on forest height, or even the height of grass in pastures. As the lidar passes over an eroded footpath that still affects the topography, the pathway's indentation is recorded by the laser beam. The lidar data can be processed to reveal tree height as well as elevation, slope, aspect, and slope length of ground features. Lidar can also be used to penetrate water to measure the morphology of coastal water, detect oil forms, fluorescent dye traces, water clarity, and organic pigments including chlorophyll. In this case, part of the pulse is reflected off the water surface, while the rest travels to the water bottom and is reflected. The time elapsed between the received impulses allows for a determination of water depth and subsurface topography.

Synthetic Aperture Radar (SAR):
SAR beams energy waves to the ground and records the energy reflected. Radar is sensitive to linear and geometric features on the ground, particularly when different radar wavelengths and different combinations of the horizontal and vertical data are employed. Different wavelengths are sensitive to vegetation or to ground surface phenomena. In dry, porous soils, radar can penetrate the surface. In 1982, radar from the space shuttle penetrated the sand of the Sudanese desert and revealed ancient watercourses. Using airborne radar in Costa Rica, prehistoric footpaths have been found.

Microwave Radar:
Beaming radar pulses into the ground and measuring the echo is a good way of finding buried artifacts in arid regions (water absorbs microwaves). Man-made objects tend to reflect the microwaves, giving one a "picture" of what is underground without disturbing the site.

Selected Papers

"Remote Sensing Methods," In Advances in Science and Technology for Historic Preservation, edited by Ray Williamson. Plenum Press. (In Press).

"Remote Sensing," In American Journal of Archeology, 99:83-84, 1995.

"Applications of Ecological Concepts and Remote Sensing Technologies in Archeological Site Reconnaissance," with F. Miller and D. Lee. (In Applications of Space-Age Technology in Anthropology, edited by Clifford Behrens and Thomas Sever. NASA, Stennis Space Center, MS, 1991.)

"Remote Sensing," Chapter 14 of Benchmarks In Time and Culture: Introductory Essays in the Methodology of Syro-Palestinian Archeology. Scholars Press. March, 1988.

"Cultural and Ecological Applications of Remote Sensing." Final Report of a Conference Sponsored by the National Science Foundation. With Daniel Gross and Paul Shankman. University of Boulder Colorado, Boulder. April, 1988.

"Conference on Remote Sensing: Potential for the Future." NASA, Stennis Space Center, Science and Technology Laboratory, SSC, MS., January, 1985.

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