Reduced level

Relative level in surveying refers to equating elevations of survey points with reference to a common assumed datum. It is a vertical distance between survey point and adopted datum plane. Thus it is considered as the base elevation which is used as reference to reckon heights or depths of other important places. Reduced here means equating and Level means Elevations. Datum may be real or imaginary location with a nominated elevation of straight zero.
Datum used.
The most common and convenient datum which is internationally accepted is mean sea level. Countries take their nearby sea levels as datum planes for calculations of Reduced levels . For example, Pakistan takes sea near Karachi as its datum while India takes sea near Mumbai as its datum for calculation of Reduced levels of different places in their respective countries. The term Reduced Level is denoted shortly by ‘RL’. National survey departments of each country determines RL’s of significantly important locations or points. These points are called as "permanent benchmarks" and this survey process is known as Great Trigonometrical Surveying (GTS). The permanent bench marks act as reference points for determining RL’s of other locations in a particular country.
Instruments.
The instruments used to determine reduced level include:-
and Coke’s reversible level. Anyone of these can be used at a time for the purpose.
RL calculation.
RL of a survey point can be determined by two methods:-