The base‐point used as the main reference station in setting out a grid for recording archaeological excavations or field survey data. The horizontal position of the datum is usually determined in latitude and longitude and with reference to the local mapping grid. The vertical dimension is conventionally given as a height above sea level, often calculated from the national standard. In remote areas the use of Global Positioning System equipment allows the rapid determination of accurate locational details for a site datum. See also Ordnance Datum.