Levelling
Topic 2 : Levelling
How levelling is conducted In the figure below the height (or reduced level) of A is known and the heights of B and C need to be estimated.
To determine the height of point B a level is set up at position I1 is between A and B. staff readings R1 and R2 are taken. Since the reduced level of A is known (RLA) the height of the line of sight or height of the plane collimation (HPC) at I1 is:
HPC at I1 = RL A + R1
To obtain the reduced level at B (or RLB):
RLB = HPC - R 2 = (RL A + R1 ) - R 2 = RL A + (R1 − R 2 )
The direction of the levelling is from A to B. Reading R1 is taken with the level facing in the opposite direction, for this reason it is called a back sight (BS). Reading R2 is taken with the level facing in the direction from A to B, for this reason it is called a fore sight (FS). The change in height from A to B, in magnitude and sign, is given by the difference of the two staff readings. Because R1 is bigger than R2, (R1 – R2) is positive and is know as a rise. To obtain the height of C the level is moved to a new position I2. A back sight is taken at B (R3) and a foresight at C (R4) the reduced level of C is:
RLc = RL B + (R 3 − R 4 )
In this case R4 is bigger than R3 , (R3 - R4) is negative and is known as a fall. At point B, both a FS (R2) and a BS (R3) have been taken from different instrument positions this is called a change point. When calculating a rise or fall this is always given by (back sight – foresight). If this is positive a rise is obtained and if negative a fall is obtained. A back sight is the first reading taken after the level has been set up, a foresight is the last reading taken at an instrument position. Any readings taken between a back sight and a foresight are known as intermediate sights.
Example
1. The level is set up at some convenient position I1 and BS of 2.191m is taken to TBM 1. 2. The staff is then moved to points A and B in turn and intermediate sights of 2.505m and
How levelling is conducted In the figure below the height (or reduced level) of A is known and the heights of B and C need to be estimated.
To determine the height of point B a level is set up at position I1 is between A and B. staff readings R1 and R2 are taken. Since the reduced level of A is known (RLA) the height of the line of sight or height of the plane collimation (HPC) at I1 is:
HPC at I1 = RL A + R1
To obtain the reduced level at B (or RLB):
RLB = HPC - R 2 = (RL A + R1 ) - R 2 = RL A + (R1 − R 2 )
The direction of the levelling is from A to B. Reading R1 is taken with the level facing in the opposite direction, for this reason it is called a back sight (BS). Reading R2 is taken with the level facing in the direction from A to B, for this reason it is called a fore sight (FS). The change in height from A to B, in magnitude and sign, is given by the difference of the two staff readings. Because R1 is bigger than R2, (R1 – R2) is positive and is know as a rise. To obtain the height of C the level is moved to a new position I2. A back sight is taken at B (R3) and a foresight at C (R4) the reduced level of C is:
RLc = RL B + (R 3 − R 4 )
In this case R4 is bigger than R3 , (R3 - R4) is negative and is known as a fall. At point B, both a FS (R2) and a BS (R3) have been taken from different instrument positions this is called a change point. When calculating a rise or fall this is always given by (back sight – foresight). If this is positive a rise is obtained and if negative a fall is obtained. A back sight is the first reading taken after the level has been set up, a foresight is the last reading taken at an instrument position. Any readings taken between a back sight and a foresight are known as intermediate sights.
Example
1. The level is set up at some convenient position I1 and BS of 2.191m is taken to TBM 1. 2. The staff is then moved to points A and B in turn and intermediate sights of 2.505m and