Method of Levelling – Computing Reduced Level Using Rise and Fall Method

If you are working in the field of civil engineering, understanding method of levelling – computing reduced level using rise and fall 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 LEVEL USING RISE AND FALL METHOD

This method consists of determining the difference of level between consecutive points by comparing each point, after the first, with that immediately preceding it. The difference between their staff readings indicates rise or fall depending on whether the staff reading at the point is smaller or greater than that at the preceding point. The reduced level of each point is then found by adding the rise too, or subtracting the fall from, the reduced level of the preceding point.

It is to be noted that the terms ‘rise’ and ‘fall’ always refer to rise or fall from the first point to the second point, second point to the third point and not conversely.

1. It consists of determining the difference of levels between the consecutive points by comparing their staff readings.

2. Obtain the rise or fall by calculating the difference between the consecutive staff readings. Rise is indicated if the back sight is more than the foresight, and a fall if the back sight is less than the foresight.

3. Find out the reduced levels of each point by adding the rise too, or by subtracting the fall from, the reduced level of the preceding point.

Check: ΣBS – ΣFS = ΣRise = ΣFall – Last RL – First RL

ΣBS = 5.430; ΣFS = 4.235; ΣRise = 3.955; ΣFall = 2.760

Here also the reduced level of the benchmark is assumed as 100.00 and the back sight to the benchmark is taken. Then the staff is held at B and reading recorded as intermediate sight.

RL of B = BM – Fall

(As the intermediate sight at B is greater than the back sight at A)

Then RL of C = RL of B – Fall of C (Fall of C = IS at C – IS at B)

Now there is a change point and so the back sight and foresight are to be taken, and as the intermediate sight (i.e., 2.540) is greater than foresight (i.e., 0.385) there is a rise.

Rise = 2.540 – 0.385 = 2.155

RL of D = RL of C + Rise = 98.125 + 2.155 = 100.280

Now there is another change point and here also, as the back sight (3.200) is greater than the F.S (1.400), there is a rise.

Rise = 3.200 – 1.400 = 1.800

RL of E = RL of D + Rise = 100.280 + 1.800 + 102.080

Intermediate sight is taken with the staff at F and here as the intermediate sight (i.e., 2.000) is greater than backsight (i.e., 1.565) there is a fall.

Fall = 2.000 – 1.565 = 0.435

RL of F = RL of E – Fall = 102.080 – 0.435 = 101.645

This procedure is repeated till the last point is reached and the usual checks are applied to assess the correctness of the calculations.

Check: ΣBS – ΣFS = ΣRise – ΣFall = Last RL – First RL = 1.195

Given are the levels recorded in a levelling book. Complete the levels using rise and fall method.

Solution: The levels are computed as below using rise and fall method

Check: ΣBS – ΣFS = Last RL – First RL = ΣRise – ΣFall

6.44 – 7.91 = –1.47, 192.15 – 193.62 = –1.47, 7.27 – 8.74 = –1.47

Hence checked.

Given are the levels recorded in a levelling book. Complete the levels using rise and fall method.

Solution: The levels are computed as below using rise and fall method

Check: ΣBS – ΣFS = Last RL – First RL = ΣRise – ΣFall

Σ6.00 – Σ3.42 = 2.58, 25.68 – 23.10 = 2.58, 2.58 – 0.00 = 2.58

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.