An instrument used in archaeology for surveying sites, especially for measuring horizontal and vertical angles. The accurate plotting of excavation trenches can be carried out and it can also be used in place of a level for determining heights and contours. There are different types of theodolite, though all include the focusing telescope, a leveling device, and scales for measuring horizontal and vertical angles. Vertical readings are easier to take on the theodolite than on the transit. The focusing telescope is mounted so that it freely rotates around horizontal and vertical axes; the telescope is usually fitted with a right-angle prism so that the observer continues to look horizontally into the eyepiece, whatever the variation of the elevation angle. Theodolites are frequently used in archaeology for setting out excavation units, mapping sites and environments, and for topographic mapping.