The point on an archaeological site from which all measurements of level and contour are taken. It is the reference point used for vertical and horizontal measurement. It can be chosen at random, at a place from which all or most of the site can be seen, and should be tied in to the national standard, usually sea level, by reference to the nearest survey point. Depths of features, of objects found in features, or simply contours, are leveled in with reference to the datum point, and are usually recorded as being a certain height 'below local datum'. Should variations in contour or the extent of the site prove too great for a single datum point, another can be used as long as it is leveled in with reference to the first. A site grid and excavation units are laid out or measured with reference to this point.