Q21. Which of the following instruments is used for measurement of sighting objects in a plane table survey?
Correct Answer: A. Alidade
📚 Detailed Explanation: The Alidade in Plane Table Surveying
The alidade is the sighting instrument specifically associated with plane table surveying. It enables the surveyor to both sight a target and draw the corresponding ray on the drawing sheet simultaneously.
What an alidade is:
An alidade is a straightedge ruler (typically 50–60 cm long) with either open sighting vanes at each end (plain alidade) or a small telescope (telescopic alidade). The ruling edge — the fiducial edge — is the straight edge against which rays are drawn on the sheet. When the sighting vanes or telescope cross-hairs are aligned with a target, the fiducial edge is simultaneously pointing in the direction of that target. A ray drawn along the fiducial edge represents the direction to the target on the map.
An alidade is a straightedge ruler (typically 50–60 cm long) with either open sighting vanes at each end (plain alidade) or a small telescope (telescopic alidade). The ruling edge — the fiducial edge — is the straight edge against which rays are drawn on the sheet. When the sighting vanes or telescope cross-hairs are aligned with a target, the fiducial edge is simultaneously pointing in the direction of that target. A ray drawn along the fiducial edge represents the direction to the target on the map.
Why the Other Options Are Wrong
| Instrument | What it actually does | Used in which survey |
|---|---|---|
| B — Clinometer | Measures the angle of slope (vertical angle) of the ground or a line of sight. Used for gradient measurement, not horizontal sighting or ray drawing. | Reconnaissance, slope measurement |
| C — Cross staff | An instrument for setting out right angles (perpendiculars) in the field by sighting two mutually perpendicular lines simultaneously. | Chain surveying (setting out offsets) |
| D — Prism square | A compact optical device also used to set out perpendiculars by using a prism to reflect the line of sight by exactly 90°. | Chain surveying (perpendicular offsets) |
Plane Table Instrument Kit — Complete List
| Instrument | Function |
|---|---|
| Alidade | Sighting objects and drawing rays |
| Spirit level | Levelling the table |
| Plumbing fork + plumb bob | Centering over ground station |
| Trough compass | Orientation (marking north, compass-based orientation) |
| Drawing sheet + drawing pins/clips | Recording the map |
| Plane table board + tripod | Supporting the sheet at working height |
Key Concepts for Students
- Alidade = sighting + ray-drawing in one: No other sighting instrument does this dual function. A theodolite measures angles numerically; the alidade records direction graphically. This is why the plane table is uniquely efficient for topographic mapping.
- Plain vs telescopic alidade: A plain alidade has open vane sights and is suitable for short-range work. A telescopic alidade has a telescope with stadia hairs and vertical arc, enabling simultaneous distance and elevation determination. Telescopic alidades produce much higher accuracy at greater range.
- Fiducial edge is the plotting edge: Always draw rays along the fiducial (ruling) edge, never along the other side of the straightedge. The fiducial edge is precisely machined to be a true straight line and is the only edge from which accurate rays can be drawn.