- if there is a build-up of several meters of soil above the layers the archaeologist is interested in, the price will obviously be much higher than if artifacts are found only centimeters below ground. Extensive vs. intensive survey One way to classify archaeological field surveys is to divide them into two types: intensive and extensive. The former is characterised by the complete or near-complete coverage of the survey area at a high resolution, most often by having teams of survey archaeologists walk in a systematic way (e.g. in parallel transects) over parcels of the landscape in question, documenting archaeological data such as lithics, ceramics and/or building remains. However, variations in artifact visibility related to topography, vegetation, and soil character, not to mention the imperfect detection abilities of human observers, bring into question the very concept of complete coverage. An extensive survey, on the other hand, is characterised by a low-resolution approach over targets within a study area (sometimes including hundreds of km²). Sometimes this involves a random sampling or some other kind of probability sample to gain a representative sample of the study area. Extensive surveys may be designed to target the identification of archaeological sites across a large area, whereas intensive surveys are designed to provide a more comprehensive picture of the location of sites and the nature of off-site data (e.g. field systems, isolated finds, etc.). An intensive survey is the more costly, timely, and ultimately informative of the two approaches, although extensive surveys can provide important information about previously unknown areas. Purposive vs. sampling survey Archaeological field surveys can also be characterized as either purposive or sampling surveys. The former, sometimes also called "archaeological prospection", involves cases where archaeologists are searching for a particular site or a particular kind of archaeological material. For example, they might be searching for a particular shipwreck or an historic fort whose exact location is no longer certain. However, they may also be searching for archaeological materials in particular locations to test hypotheses about past use of those spaces. Sampling surveys, on the other hand, have the goal of obtaining a representative sample of some population of sites or artifacts in order to make generalizations about that population. This involves some probability sampling of spatial units, such as random or stratified random sampling of geometrical (often square) or irregular spatial units. Fieldwalking (transects) Conventionally, fieldwalking in grids or along lines called