Archeological Mapping

Before any excavation is begun at a site, the archeologist must prepare a survey map of the site. Site mapping may be as simple as a sketch of the site boundaries, or as complex as a topographic map complete with details about vegetation, artifacts, structures, and features on the site. By recording the presence of artifacts on the site, the site map may reveal information about the way the site was used, including patterns of occupational use. Contour maps may shed light on ways in which more recent environmental activity may have changed the original patterns of use. In cases where structural remains are visible at a site, the site map can provide a basis for planning excavations.

When staking out a site to be excavated, the archeologist typically lays out a square grid that will serve as a reference for recording data about the site. The tools required to construct the grid may be as simple as a compass, a measuring tape, stakes, and a ball of twine. After the grid has been laid out, the archeologist draws a representation of it on graph paper, being careful to note the presence of any physical landmarks such as trees, rivers, and large rocks. Once the excavation is underway, each artifact recovered is mapped into the square in the grid, and layer in which it was found.

As artifacts are removed from each layer in the site, their exact positions are plotted on a map. At the end of the excavation, a record of the site will exist in the form of maps for each excavated layer at the site. Photographs are also taken of each layer for comparison with the maps.

To facilitate artifact recovery, deposited material at the site may be screened or sifted to make sure materials such as animal bones, snails, seeds, or chipping debris are not overlooked. When a screen is used, shovelfuls of soil are thrown on it so that the dirt sifts through the screen, leaving any artifacts behind. In some cases, the deposit may be submerged in a container filled with plain or chemically-treated water. When the water is agitated, light objects such as seeds, small bones, and charred plant material rise to the top of the container.

Prior to shipment to the laboratory for processing, artifacts are placed in a bag that is labeled with a code indicating the location and stratigraphic layer in which the artifacts were found. Relevant information about each artifact is recorded in the field notes for the site.

Many mapping techniques developed for use on land have also been adapted for underwater archeology. Grids can be laid out to assist in mapping and drawing the site, and to assist the divers who perform the excavation. In this case, however, the grids must be weighted to keep them from floating away, and all mapping, recording, and photographing must be done with special equipment designed for underwater use.

Most modern archeologists will attempt to place data taken from a site into archeological context by mapping the spatial and stratigraphic dimensions of the site.

Spatial dimensions include the distribution of artifacts, and other features in three dimensions. The level of detail given in the spatial description typically depends on the goals of the research project. One hundred years ago, finds were recorded much less precisely than they are today; it might have been sufficient to map an object's location to within 25 sq yd (7 sq m). Today, the location of the same artifact might be recorded to the nearest centimeter. Modern archeologists still use maps to record spatial information about a site. Such information includes the spatial distribution of artifacts, features, and deposits, all of which are recorded on the map. Measuring tools range from simple tapes and plumb bobs to highly accurate and precise surveying instruments called laser theodolites.

The accuracy of a map is the degree to which a recorded measurement reflects the true value; the precision of the map reflects the consistency with which a measurement can be repeated. Although the archeologist strives for accuracy in representing the site by the map, the fact that much of what is recorded represents a subjective interpretation of what is present makes any map a simplification of reality. The levels of accuracy and precision that will be deemed acceptable for the project must be determined by the archeologists directing the investigation.

The second technique involved in recording the archeological context of a site is stratigraphic mapping. Any process that contributed to the formation of a site (e.g., dumping, flooding, digging, erosion, etc.) can be expected to have left some evidence of its activity in the stratification at the site. The sequential order these processes contribute to the formation of a site must be carefully evaluated in the course of an excavation. The archeologist records evidence of ordering in any deposits and interfaces found at the site for the purposes of establishing a relative chronology of the site and interpreting the site's history. In order to document the stratification at the site, the archeologist may draw or photograph vertical sections in the course of an excavation. Specific graphing techniques have been developed to aid archeologists in recording this information. Finally, the archeologist typically notes such details as soil color and texture, and the presence and size of any stones, often with the aid of reference charts to standardize the descriptions.

Although all archeologists agree that keeping careful records of an excavation is essential to good practice, there is a certain amount of disagreement as to what constitutes archeological data. Many of the practices of the eighteenth century archeologist seem crude when compared to the detailed site information that is now considered vital—for example, the exact positioning and magnetic properties of fired clay. However, the practices of today will no doubt seem coarse to the archeologist of the next century.