How to calculate RL in surveying?

RL may be defined as the elevation of the survey station’s points which is commonly supposed with reference to the datum. It is a vertical distance below the datum surface. Mean sea level is the most common datum used for surveying. The RL term is most commonly used in the levelling process.

Methods of calculating RL (reduced level):

There are two methods to calculate the RL:

1. Height of instrument method
2. Rise and fall method

#1. Height of instrument method:

This process consists of the elevation of the surface of collimation (H.I) for each set up of instrument and then we can obtain the reduced level of the points with the reference to the plan of collimation. The is done by the following process:-
First of all, the height of the instrument is determined by adding the back sight of the RL of the beginning point.

Then the reduced level of the intermediate point and the first change point can be obtained by subtracting the respective reading from the H.I
When the instrument is shifted a new plane of collimation is set up and the height is calculated by adding back sight reading to the first change point RL.

The reduced level of the successive points and second change point is calculated by subtracting their staff readings from the new H.I
This process is repeated until the R.L is worked out. then the arithmetical check is applied
Arithmetical check: the difference between the sun of the back sight and the sum of foresight should be equal to the difference of the 1st RL and last RL.

#2. Rise and fall method:

In this method, the difference between the points is calculated by comparing each point after the first. The difference between the staff readings shows the rise and fall of that point. Then the R.L of each point is obtained by adding rise and subtracting fall.

Arithmetical Check :

There are three checks in this process to check the accuracy of the R.L
The difference between the sum of the back sights and the sum of the foresight is equal to the difference of the sum of the rises and that of the falls is equal to the difference between the first R.l and last R.L.

Also Read: Rise and fall method with Examples