Q24. The instrument used for accurate centering in plane table survey is:
Correct Answer: C. Plumbing fork
📚 Detailed Explanation: Accurate Centering Using the Plumbing Fork
Centering is the operation of positioning the station point on the drawing sheet directly over the physical ground station peg. The plumbing fork is the dedicated instrument for this task because it provides both the upper reference point (touching the sheet) and the lower suspension point for a plumb bob.
How accurate centering is achieved:
The plumbing fork is placed with its upper arm touching the station mark on the drawing sheet. A plumb bob hangs from its lower arm. The tripod legs are adjusted (or the table shifted) until the plumb bob aligns vertically with the ground station peg. When the plumb bob is directly above the peg, the station mark on the sheet is vertically above the peg — centering is complete. The accuracy depends on how precisely the plumb bob aligns with the peg, which requires care on windy days.
The plumbing fork is placed with its upper arm touching the station mark on the drawing sheet. A plumb bob hangs from its lower arm. The tripod legs are adjusted (or the table shifted) until the plumb bob aligns vertically with the ground station peg. When the plumb bob is directly above the peg, the station mark on the sheet is vertically above the peg — centering is complete. The accuracy depends on how precisely the plumb bob aligns with the peg, which requires care on windy days.
Why the Other Options Are Wrong
| Option | Actual function | Setup operation performed |
|---|---|---|
| A — Spirit level | A glass vial containing bubble and liquid; bubble moves to centre when horizontal | Levelling (making the table horizontal), not centering |
| B — Alidade | Straightedge sighting ruler with vane sights or telescope and fiducial edge | Sighting objects and drawing directional rays (observation), not centering |
| D — Trough compass | Elongated magnetic compass that fits alongside the north line on the sheet | Orientation by magnetic north, not centering |
Quick Reference: Instrument → Setup Operation
| Setup operation | Instrument used |
|---|---|
| Levelling | Spirit level (tubular or circular) |
| Centering | Plumbing fork + plumb bob |
| Orientation | Trough compass (first station) or alidade (back-sighting) |
| Sighting / ray drawing | Alidade (plain or telescopic) |
Key Concepts for Students
- Plumbing fork is for centering, spirit level is for levelling: This pairing is one of the most common exam test points. Both instruments are part of the plane table kit, and both are used at each station setup, but they serve completely different operations.
- Centering accuracy depends on scale: At small scales (e.g., 1:5000), a centering error of even 30 cm appears as 0.06 mm on the sheet — negligible. At large scales (e.g., 1:500), the same error is 0.6 mm — a significant plotted error. Precise centering with the plumbing fork is therefore most important for large-scale surveys.
- Wind affects centering accuracy: The plumb bob string oscillates in wind, making it hard to judge the exact point below. In windy conditions, experienced surveyors dampen the plumb bob in a bucket of water or use a shorter string to reduce swing. This is a practical field consideration unique to plumbing-fork centering.