the level

 

 A level essentially comprises a telescope rotatable about a vertical axis; it is usued to create a horizontal line of sight so that height differences can be datermined and stakeouts can be .performed

the leica Geosystems levels are also equipped with a horizontal circle that is very useful for 

setting out right angles, these levels can be used to determine distances optically withan accuracy to 0. 1 - 0.3 metres.