A bench mark (BM) in surveying is defined as:
Correct Answer: A. A fixed point of known elevation
📚 Detailed Explanation: Bench Mark = Fixed Point of Known Elevation
Why A (A fixed point of known elevation) is correct: A Bench Mark (BM) is a permanent, well-defined physical object (a bronze disk, a chiselled mark on a stable structure, a concrete pillar) whose elevation (Reduced Level) above the datum has been precisely determined and is officially recorded.
Types of Bench Marks in India
| Type | Established by | Accuracy / Spacing |
|---|---|---|
| GTS Benchmarks (Great Trigonometric Survey) | Survey of India | Primary; highly precise; spaced ~32 km apart along major routes |
| Permanent Benchmarks | Survey of India / State agencies | Secondary; spaced ~5 km; marked on stable structures (bridges, buildings) |
| Arbitrary Benchmarks | Engineer in charge of a project | Tertiary; used for local project control; elevation assumed or tied to GTS |
| Temporary Benchmarks (TBM) | Field surveyor | Used during construction; tied to a permanent BM; removed after project |
Distinction from Datum: A datum is the reference surface (MSL); a bench mark is a specific physical point with a known RL above that datum. Bench marks allow surveyors to start (and close) levelling runs without measuring from MSL every time.
- Bench Mark = a fixed physical point of known elevation (Reduced Level) above datum.
- Used as the starting or closing point for levelling surveys.
- GTS benchmarks are the primary control network in India, referenced to Mumbai MSL.