Reduced level (RL) is a term used in surveying to equate elevations of survey points with reference to a common assumed vertical datum. It is a vertical distance between a survey point and the adopted datum surface. The most common datum used is Mean Sea Level. The RL is used as a base elevation to reckon heights or depths of other important places. The process of determining RL involves taking levels across relevant parts of the building site using equipment such as optical leveling instruments like automatic level, Y level, dumpy level, or Cokes reversible level, and a leveling staff. The instruments are used to determine the elevation of the plane of collimation, which is then used to obtain the reduced levels of the points with reference to the respective plan of collimation. There are two methods to calculate RL: the height of instrument method and the rise and fall method. The height of instrument method involves finding the elevation of the plane of collimation for every setting of the instrument and then obtaining the reduced levels of the points with reference to the respective plan of collimation. The rise and fall method involves finding the difference between the sum of back sights and sum of fore sights, which should be equal to the difference of the first and last RLs. The RLs of different points are obtained for construction of buildings, roads, and dams, and the ground surface is then leveled to the RL, which is obtained by taking the arithmetic mean of RLs of different points.