Q12. What is the function of the plumbing fork in plane table surveying?
Correct Answer: A. Used for centering of plane table
📚 Detailed Explanation: The Plumbing Fork & Its Role in Centering
The plane table setup kit has a dedicated instrument for each of the three setup operations. The plumbing fork (also called the U-fork) is the specific tool assigned to the centering operation.
Fig: The U-shaped plumbing fork. Its upper point rests on the station mark on the sheet; the lower arm suspends a plumb bob that must align vertically with the ground peg below.
How the plumbing fork works:
The plumbing fork is a U-shaped (or L-shaped) metal frame with two arms at different heights. The upper arm rests against the drawing sheet at the station mark. The lower arm extends below the table and carries a plumb bob on a thread. When the plumb bob hangs directly over the physical ground station peg (tack or nail), the point representing the station on the sheet is vertically above the actual peg — the table is correctly centered.
The plumbing fork is a U-shaped (or L-shaped) metal frame with two arms at different heights. The upper arm rests against the drawing sheet at the station mark. The lower arm extends below the table and carries a plumb bob on a thread. When the plumb bob hangs directly over the physical ground station peg (tack or nail), the point representing the station on the sheet is vertically above the actual peg — the table is correctly centered.
Why the Other Options Are Wrong
| Option | Correct instrument | Why the plumbing fork does not do this |
|---|---|---|
| B — Levelling | Spirit level (circular or tubular) | Levelling makes the table horizontal, which has nothing to do with alignment above a ground peg. The spirit level bubble is used for this purpose. |
| C — Orientation | Trough compass or alidade (for back-sighting) | Orientation rotates the table to align lines on the sheet with ground lines. The plumbing fork cannot detect angular alignment. |
| D — Sighting objects | Alidade (with sighting vanes or telescope) | The alidade is the straightedge ruler used to sight objects and draw rays. The plumbing fork is used solely for vertical alignment, not for sighting. |
Key Concepts for Students
- One instrument per operation: Spirit level → levelling; Plumbing fork + plumb bob → centering; Trough compass or alidade → orientation; Alidade → sighting. Memorise this mapping — it appears frequently in multiple-choice questions on plane table accessories.
- Centering matters more at large scales: The position error from off-center setup = (ground eccentricity) × (map R.F.). At small scales (1:10,000) even 50 cm of eccentricity appears as 0.05 mm on the sheet — negligible. At large scales (1:500) the same 50 cm error becomes 1 mm — significant. Precise centering with the plumbing fork is therefore critical for large-scale detail surveys.
- Plumbing fork vs. plumb bob: The plumb bob alone (a weighted string) can hang below the table, but it cannot align with the station mark on the sheet without the fork holding the upper reference point. The fork provides both the upper reference (touching the sheet mark) and the lower suspension (for the plumb bob). Both together achieve centering.