Method of Levelling – Computing Reduced Levels Using Height of Collimation Method (hi Method)

If you are working in the field of civil engineering, understanding method of levelling – computing reduced levels using height of collimation method (hi method) is absolutely essential. In this comprehensive guide, we will walk you through everything you need to know about this topic, from the basic concepts to practical applications on the construction site.

COMPUTING REDUCED LEVELS USING HEIGHT OF COLLIMATION METHOD (HI METHOD)

In this method, the reduced levels of points are computed by calculating the reduced levels of the plane of collimation for each set up of the instrument.

The height of collimation is obtained by adding the staff reading, which must be back sight to the known reduced level of the point on which the staff stands. Reduced levels of all the other points are obtained by subtracting the staff reading from the height of collimation. When the instrument is changed to a new station, a new height of collimation is obtained by adding the new backsight with the reduced level of the last point obtained from the previous set up of the instrument. The steps involved in booking and reading the level in the height of the collimation method are illustrated with the help of an extract from a field book given below.

In this table, the back sight, intermediate sight and foresight are the readings taken in the field and in the remarks column benchmarks and change points are specified. Then, with the help of this available data the height of the instrument and reduced levels are calculated and the usual checks are carried out.

1. To begin with, the elevation of the plane of collimation of the first instrument station (I1) is to be calculated. This is done by adding the back sight to the reduced level of the benchmark. The reduced level of the benchmark, i.e., point A, is taken as 100.00 and backsight is taken by holding a staff at point A (i.e., 0.665).

Elevation of the plane of collimation = 100 + 0.665 = 100.665

This is entered as the height of the instrument (HI) as seen in the table.

2. Then, from the same instrument station, readings are taken of the staff held at B and C and entered as the intermediate sight. Then the reduced level of B and C are calculated.

For example, RL of B = HI – IS = 100.665 – 0.825 = 99.840

3. Now the instrument is shifted to the next point I2 and the backsight and foresight are taken. Then, the HI and reduced level at this point is calculated.

RL = HI at first point – FS

= 100.665 – 0.385

HI = RL + BS

= 100.280 + 3.200

5. Now the checks are carried out to ascertain the correctness of the readings. i.e., ΣBS – ΣFS = Last RL – First RL

ΣBS = 5.430; ΣFS = 4.235; First RL = 100.00; Last RL = 101.195

5.430 – 4.235 = 101.195 – 100 = 1.195

Hence checked.

Complete the levelling table given below

Solution: The levels are computed as below

Check: ΣBS – ΣFS = Last RL – First RL

ΣBS = 6.44; ΣFS = 7.91; Last RL = 192.15; First RL = 193.62

i.e., 6.44 – 7.91 = –1.47 m

192.15 – 193.62 = –1.47 m

Hence checked.

Compute the levels from the levelling field book using HI method.

Solution. The levels are computed as below.

Check: ΣBS – ΣFS = Last RL – First RL

ΣBS = 6.00; ΣFS = 3.42; Last RL = 25.68; First RL = 23.10

i.e., 6.00 – 3.42 = 2.58 m

25.68 – 23.10 = 2.58 m

Hence checked.

Here, in this problem, the readings have to be entered in a book form, entries are checked and the reduced levels found.

Check: ΣBS – ΣFS = Last RL – First RL

ΣBS = 4.565; ΣFS = 5.523; Last RL = 38.605; First RL = 39.563 4.565 – 5.523 = –0.958

38.605 – 39.563 = –0.958

Hence checked.

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Conclusion

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❓ Frequently Asked Questions (FAQ)

Why is surveying important in construction?

Surveying is critical in construction because it helps establish accurate measurements, positions, and levels on the ground. Without proper surveying, structures could be misaligned, leading to costly errors and structural issues.

What are the most common surveying instruments?

The most commonly used surveying instruments include the auto level, theodolite, total station, GPS equipment, chain, tape measure, and ranging rods. The choice of instrument depends on the type and precision of the survey required.