In surveying, RL stands for Reduced Level, which refers to equating 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, which is usually mean sea level. The instruments used to determine reduced level include optical leveling instruments like automatic level, Y level, dumpy level, or Cokes reversible level, and a leveling staff. The RL of a survey point can be determined by two methods: the height of instrument method and the rise and fall method. The significance of RL is that it is used to ensure proper drainage of water from roads and other structures. National survey departments of each country determine RLs of significantly important locations or points, which are called permanent benchmarks, and these benchmarks act as reference points for determining RLs of other locations in a particular country.