In lieu of an abstract, here is a brief excerpt of the content:

12 Ground Truthing the Results of Geophysical Surveys MichaelL.Hargrave Spectacular images of submound structures, Plains pithouse villages, and Spanish missions may convey the impression that most “good” geophysical surveys, like latenight television, leave little to the imagination. This impression is a result of an understandable tendency for geophysicists to distribute images from their most dramatic surveys. In fact, well-executed surveys often yield useful results that are not immediately interpretable by many archaeologists. Effective ground truthing is often the key that unlocks the information content of a geophysical map. Ground truthing is an effort to verify and enhance the results of a remote sensing study through the use of independent evidence. Note that the word truthing refers to the interpretation of the remote sensing data; it does not imply that the actual data may be spurious. If a remote sensing study is properly executed, variation in the data other than statistical noise will have some cultural or geological source or sources. The origins of ground truthing are, of course, intertwined with those of remote sensing . Remote sensing began with, or was at least greatly stimulated by, the development of photography. In the form of aerial photography, remote sensing was first used systematically during World War I (Scollar et al. 1990:26). Archaeological features were sometimes detected during the course of military reconnaissance missions. The recognition that remotely sensed military information could be extremely useful must have led to an almost immediate concern with verifying the reliability of that information. Soon after the war, aerial remote sensing for archaeological purposes began in earnest (Crawford 1924). 270 ~ Michael L. Hargrave In archaeogeophysics, ground truthing generally focuses on determining the subsurface sources of geophysical anomalies. Nonarchaeological information that can be used to ground truth geophysical data includes current and historic maps and photographs (particularly aerial photographs), other historical documents, and anecdotal information provided by local informants. These sources are generally most useful in identifying relatively recent phenomena that may account for geophysical anomalies: historic buildings, roads, drainage features, and other landscape modifications. Archaeologists generally view excavation as the preferred means of ground truthing a geophysical survey. Unfortunately, most of the available overviews of geophysics that are directed at archaeologists provide little or no discussion of the merits of various approaches to archaeological ground truthing. This may reflect or at least perpetuate the assumption that effective ground truthing is a straightforward matter that requires no particular consideration. In fact, the potential information return of a geophysical study is far too commonly greatly limited as a result of insufficient or ineffective ground truthing. The goal of this chapter is to identify some of the important issues in ground truthing and to discuss a number of approaches that have been found useful by the authors of this volume. Why Ground Truth? To explain why ground truthing is so important, it is useful to review the goals of a geophysical survey. At the most basic level, the archaeologist is generally interested in identifying subsurface cultural features. In remote sensing terms, this goal involves both detection and classification. One must first detect anomalies that may be associated with discrete subsurface phenomena. Classification begins with an attempt to differentiate anomalies associated with cultural features from clutter, i.e., anomalies associated with other phenomena. Clutter can include anomalies related to tree roots, rodent burrows, rocks, plow furrows, recent metallic debris, and so forth. Under favorable circumstances, the goals of classi fication can be more ambitious. Here one may attempt to classify anomalies into useful archaeological categories such as pits, house basins, wall trenches, and graves. The preceding chapters in this volume have presented a number of maps in which this type of classification can be achieved with minimal excavation. These maps result from surveys that were well executed under favorable conditions. At many sites, however, classification is seriously complicated by the issue of equifinality. Very different phenomena can be manifested in a geophysical map by very similar anomalies. Consequently, a geophysical map cannot necessarily be interpreted as if it were an aerial photograph of a site from which the A horizon has been removed to expose the subsurface features. The size and shape of a geophysical anomaly may or may not resemble the dimensions of the underlying subsurface deposit. The factors that influence the spatial relationship between an anomaly and its source include the geometry and material composition of the source, the nature [18...

Share