Deduction of Levels / Reduced Level (RL)

Deduction of Levels / Reduced Level (RL)

In levelling, determining the elevation of points is a fundamental task. The elevation of any point with respect to a reference level (datum) is known as the Reduced Level (RL). In ITI Draughtsman Civil, students must understand how to calculate or deduce reduced levels accurately using different methods.

1. Definition of Reduced Level (RL)

Reduced Level (RL) is the vertical distance of a point above or below a reference surface called datum, usually taken as mean sea level (MSL).

• If a point is above datum → RL is positive.
• If a point is below datum → RL is negative.

2. Terms Used in Levelling

• Back Sight (BS): First reading taken on a known point.
• Fore Sight (FS): Last reading before shifting the instrument.
• Intermediate Sight (IS): Readings taken between BS and FS.
• Height of Instrument (HI): Height of line of sight above datum.

3. Methods of Deduction of Reduced Levels

There are two main methods used to calculate reduced levels:

(a) Height of Instrument (HI) Method

In this method, the height of the instrument is calculated first, and then RLs of other points are determined.

Formula

• HI = RL of Benchmark + Back Sight (BS)
• RL = HI – Staff Reading (IS or FS)

Procedure

1. Take BS on a known RL (Benchmark).
2. Calculate HI.
3. Subtract staff readings from HI to get RL of points.

Advantages

• Simple and quick method.
• Suitable for field calculations.

(b) Rise and Fall Method

In this method, the difference between consecutive readings is used to determine whether there is a rise or fall in level.

Formula

• Rise = Previous Reading – Next Reading (if positive)
• Fall = Next Reading – Previous Reading (if positive)
• RL = Previous RL + Rise – Fall

Procedure

1. Compare consecutive staff readings.
2. Calculate rise or fall.
3. Add rise or subtract fall from previous RL.

Advantages

• More accurate method.
• Provides complete check of calculations.

4. Arithmetic Check

To ensure accuracy, the following check is applied:

• Sum of BS – Sum of FS = Last RL – First RL

If both sides are equal, calculations are correct.

5. Importance of Reduced Levels

• Used in preparation of contour maps.
• Helps in construction of roads and buildings.
• Essential for drainage and irrigation design.
• Used for calculating earthwork.

6. Precautions

• Take accurate staff readings.
• Ensure proper leveling of instrument.
• Record data carefully in field book.
• Perform arithmetic checks.

Role of Draughtsman Civil

A Draughtsman Civil uses reduced levels to prepare profiles, sections, and contour drawings. Accurate RL calculation ensures proper planning and execution of construction projects.

Conclusion

Deduction of reduced levels is a key process in levelling. Methods like HI method and Rise & Fall method are used to calculate elevations accurately.

For ITI Draughtsman Civil students, understanding RL calculation is essential for accurate surveying and successful construction work.