A system of perpendicular lines and equally spaced points to form a rectangle which is used as a frame of locational reference on an archaeological sites. A grid is usually defined by its distance and direction in reference to a datum point. Excavations units are often planned and recorded by grid. Grids are often aligned with either the anticipated site layout or with a landform upon which the site sits. Many archaeological sites are surveyed by measuring from a grid enclosing the site. It is a rectilinear system of X, Y coordinates which is established over the area to be excavated so that spatial control can be maintained.