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.[1] Thus it is considered as the base elevation which is used as reference to reckon heights or depths of other important places.[2] Reduced here means equating and Level means Elevations. Datum may be real or imaginary location with a nominated elevation of straight zero.[3]

## 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.[4][5] [6] [7]

## Instruments

The instruments used to determine reduced level include:-

• Optical Levelling instruments like Automatic Level, Y-Level, Dumpy level and Coke’s reversible level. Anyone of these can be used at a time for the purpose.
• Levelling Staff.
• Tripod stand.[8]

## RL calculation

RL of a survey point can be determined by two methods:-

1. Height of Instrument method
2. Rise and Fall method

## Significance

1. For drainage of water under gravity a suitable slope is required. Thus roads are built in the fashion that their RL’s on sides are comparatively smaller than the RL at the mid-span of the road. This ensures proper drainage of water from roads.
2. For construction of buildings, roads, dams a horizontal levelled surface is required. So, at construction sites, RLs of different points are obtained. The ground surface is then being levelled to the RL which is obtained by taking arithmetic mean of RLs of different points.[9]

## References

1. ^ "Reduced Level explanation" (PDF). Retrieved 17 October 2014.
2. ^ Tides and Currents. http://tidesandcurrents.noaa.gov/datum_options.html
3. ^ "Reduced Level definition". Retrieved 17 October 2014.
4. ^ "Datum of India and Pakistan". Retrieved 17 October 2014.
5. ^ "Datum Surface". Retrieved 17 October 2014.
6. ^ "Tides and Currents". Retrieved 17 October 2014.
7. ^ "GTS benchmarks of India and Pakistan". Retrieved 15 November 2014.
8. ^ "Instruments used to determine reduced level". Retrieved 17 October 2014.
9. ^ Dr. B.C Punmia and Ashok Kumar Jain. (2014). "Levelling". A text book of Surveying Vol. 1.