An instrument used in surveying which takes vertical measurements and which is much used in excavation for the recording of site contours and accurate depths of features, especially for making maps and identifying the location of artifacts. There are several types of leveling instrument, the Y or dumpy level, the tilting level, and the self-leveling level. Each consists of a telescope fitted with a spirit level and, generally, mounted on a tripod. It is used in conjunction with a graduated rod placed at the point to be measured and sighted through the telescope. The theodolite (q.v.), or transit, is used to measure horizontal and vertical angles; it may be used also for leveling. The differences between the types are in the ease of leveling: the first has a single spirit level for the whole instrument, the second a separate spirit level for spindle and telescope with a tilting mechanism and adjustable screw on the telescope, and the third an optical part operated by a pendulum so that the line of sight is always horizontal. Having established a datum point, the instrument is sighted on a leveling staff or rod which is marked in a graduated scale, metric, or imperial. The difference in level between the telescope and the base of the rod can be read off on this scale, and the result subtracted from the height of the level itself above ground; the final figure gives the real height, or depth, of the feature above or below the ground at instrument point. Subtracting the stadia rod reading from the height of the level above the ground surface gives the difference in height between ground surface at the instrument station and the ground surface at the datum point. A series of levels taken across a site will give contours, while excavated features and small finds can be leveled in with greater accuracy than with tapes from a hypothetical ground surface. The term is also used to refer to the actual height measurements taken with such an instrument. More generally, archaeologists often use the term 'level' interchangeably with layer. In excavations the remains are divided into levels that contain the buildings and objects belonging to a phase.