Section 4: Soil and Water Conservation Engineering
Surveying and Levelling
Subtopic: Types of levelling
Question Statement:
The underside beam of a railway bridge, a permanent Bench Mark with RL 85.168 m, is used for a leveling operation. The Back Sight (staff held inverted against the BM) = 3.645 m. For a Fore Sight of 1.523 m at a construction-site point, find its RL (m).
Official Answer Key:
80 to 80
Step-by-Step Solution & Explanation
• **Concept (Inverted Staff Leveling):**
- When a leveling staff is held upside down against the underside of an elevated overhead structure (e.g., bridge girder, ceiling), the staff reading is negative relative to the instrument line of sight.
- Height of Instrument ($HI$) $= \text{BM RL} - (\text{Inverted Back Sight}) = 85.168 - 3.645 = 81.523\text{ m}$.
• **Reduced Level ($RL$) at Ground Point:**
- The Fore Sight on the normal ground point is positive: $FS = 1.523\text{ m}$.
$RL = HI - FS = 81.523\text{ m} - 1.523\text{ m} = 80.000\text{ m}$
• **Conclusion:** The reduced level of the construction site point is exactly $80.00\text{ m}$ (Official range: 80 to 80).
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