When most people think of archaeology, they think of excavations and digging. In fact, excavation is only a single element of the archaeological process. The process of selecting an archaeological site usually begins with "armchair work." This is preliminary research using maps and other documents to try to find a prospective site. Maps can provide clues to where buried or ruined sites may lay, and old maps sometimes facilitate the process. Documentary evidence can lead to the discovery of a site as well. Once a prospective site has been selected, a process called field walking is begun. Field walking can reveal small artifacts, such as coins, tools, and pieces of pottery. At a large site, the field is often divided among a number of people. The field walkers superficially check their area of the site in an attempt to find some small artifacts. If the field walk indicates that a worthwhile site may be present, a topographic survey of the area is often performed. Aerial photographs can reveal places where ditches were dug or walls are buried. Sometimes, for example, taller and greener plants than their surroundings extend along a line on the ground. This usually means that a ditch was dug along that line, because the dirt that filled it was richer than the surrounding dirt. Aerial photography can reveal such hidden details, but it is still a rather superficial method of examining a site. Resistivity surveys can reveal more details beneath the surface of a prospective site. This type of survey works by passing an electric current through the ground to determine resistance. Stones would be more resistant than their surroundings, because less water is present in them, whereas holes would be less resistant, because water collects in them. Magnetometers are often used, as well, to detect similar features. Metal detectors are also used, but these devices are not always accurate. Newer techniques, such as radar surveys, are now being used as well. These types of surveys are more powerful, accurate, and versatile than the others.