Entry of Level Book (Reduced Level Calculation Methods)
In levelling, all observations are recorded systematically in a level book. Proper entry of data and correct calculation of Reduced Levels (RL) are essential for accurate results. Two main methods are used for RL calculation: Height of Instrument (HI) method and Rise & Fall method. For ITI Surveyor trainees, understanding these methods is very important.
Level Book
A level book is used to record staff readings taken during levelling. It helps in calculating elevations and maintaining a proper record of survey data.
Common Terms Used
- BS (Back Sight): First reading on known point
- IS (Intermediate Sight): Reading between BS and FS
- FS (Fore Sight): Last reading before shifting instrument
- RL (Reduced Level): Elevation of a point
- HI (Height of Instrument): Height of line of sight
Format of Level Book
| Station | BS | IS | FS | HI | RL | Remarks |
|---|
1. Height of Instrument (HI) Method
In this method, the height of instrument is calculated and then RL of points is determined.
Formula
- HI = RL + BS
- RL = HI − FS (or IS)
Example Table
| Station | BS | IS | FS | HI | RL |
|---|---|---|---|---|---|
| BM | 1.50 | - | - | 101.50 | 100.00 |
| A | - | 1.20 | - | 101.50 | 100.30 |
| B | - | - | 1.80 | - | 99.70 |
Advantages of HI Method
- Simple and quick calculation
- Suitable for large number of readings
2. Rise and Fall Method
In this method, the difference between consecutive readings is calculated to determine rise or fall.
Formula
- Rise = Previous reading − Current reading
- Fall = Current reading − Previous reading
- RL = Previous RL + Rise or − Fall
Example Table
| Station | BS | IS | FS | Rise | Fall | RL |
|---|---|---|---|---|---|---|
| BM | 1.50 | - | - | - | - | 100.00 |
| A | - | 1.20 | - | 0.30 | - | 100.30 |
| B | - | - | 1.80 | - | 0.60 | 99.70 |
Arithmetic Check
To check accuracy:
- Σ BS − Σ FS = Last RL − First RL
- Σ Rise − Σ Fall = Last RL − First RL
Precautions
- Record readings clearly and immediately
- Avoid overwriting
- Check calculations carefully
- Maintain proper units
Importance in Surveyor Trade
Proper entry in level book and correct RL calculation are essential for accurate levelling. These records are used for planning, design, and construction work.
Conclusion
Level book entry is an important part of levelling. Both HI method and Rise & Fall method are used for calculating reduced levels.
For ITI Surveyor trainees, mastering these methods ensures accurate data recording and reliable survey results.