When I was asked to write an article for this issue about the use of technology in archaeology, I began to think about the myriad of systems utilized in field work each season. Between the research vessel, guidance and steering systems, the ROVs remote sensing equipment, array of video feeds, data processing systems, display graphics, etc. a wide variety of technical explanations and themes readily present themselves. What came to mind is one of the crucial questions technology is used to answer … “Where is it?”
Determining the location of a site seems simple enough, however working underwater increases the difficulty of doing so and is further complicated when working beyond diver depths. It is surprising how often archaeologists don’t know the answer to “Where is it?” Data on the location of a site should begin with one or more sets of coordinates, accurate coordinates. All too often when the time comes to re-visit a site the archaeologist begins divining the location on the surface from the positions of houses on shore, a point of land, or memories of nearby rock formations. Unfortunately, official reports are also encountered that use such location references. Without an accurate location on the surface, the location of the site lying on the seafloor below remains unknown. These situations result in a new search by systematic survey, and if the last site visit was years ago it is sometimes never found.
Differential GPS (DGPS) is a crucial component in the technology of determining location; this system tells you where you are on the earth. Although studies are ongoing for the accuracy of DGPS, it has generally been found to be accurate to within 1-2 meters. The accuracy is partly a function of distance to broadcasting stations, and given the size of the Mediterranean accuracy is typically around a meter or less. A well-maintained DGPS system will very accurately provide position information for a vessel on the sea’s surface; however, sites are on the seafloor, so several methods are typically used to determine position on the seafloor. A tried-and true method in shallow water calls for divers to raise some form of buoy to the surface on a line where its position can be determined by the research vessel. The amount of error in this method occurs in how vertically straight the line between the point to be measured and buoy can be maintained, and how carefully the distance from the DGPS receiver to the buoy is measured. The deeper the water the more difficult it is to maintain a straight line and the greater amount of error. It is usually an adequate method in shallow water when coupled with a depth reading for the point and recording accurate representation of where on seafloor the point falls. Problematically the error from the buoy line’s deviation off vertical is not consistent, nor is the buoy position calculation; hence, comparing multiple points, of scattered artifacts or the limits of a site for example, is less reliable. Once fixed absolute points are determined on the site, relative position data can be taken with a variety of methods that utilize measuring tapes, a compass, and datum points. These measurements are then processed in software to determine relative position information.