Q23. In which of the following types of surveying is the instrument alidade used?
📚 Detailed Explanation: The Alidade and Its Survey Context
The alidade is an instrument that combines a sighting device with a straightedge ruler for drawing directional rays. It is exclusively associated with plane table surveying and forms the core observation tool of the entire method.
Why Option D is Correct
In plane table surveying, the alidade is placed with its fiducial (ruling) edge touching the plotted station point on the drawing sheet. The sighting vanes (or telescope) are aligned with the target. A ray drawn along the fiducial edge then represents the direction from the station to the target on the map. The alidade makes it possible to simultaneously sight, determine direction, and draw the ray — the three core actions of plane table field work.
Why the Other Options Are Wrong
| Option | Survey instruments actually used |
|---|---|
| A — Aerial surveying | Camera, stereoscope, photogrammetric plotter, GPS. No alidade is used in aerial photogrammetry. |
| B — Tacheometric surveying | Tacheometer (a type of theodolite) or total station with stadia hairs. The instrument is mounted on a tripod and angles are read digitally or from graduated circles — no drawing sheet or alidade is used. |
| C — Chain surveying | Chain or tape, ranging rods, cross staff, offset staff. Chain surveying is entirely linear — no angle-measuring or sighting instrument is used for the main framework. |
Key Concepts for Students
- Alidade = plane table, always: Whenever “alidade” appears in an option list alongside other survey methods, the answer is plane table surveying without exception. No other survey method uses an alidade as its primary observation instrument.
- Plain vs telescopic alidade types: A plain alidade has simple vane sights and is used for basic direction work. A telescopic alidade (Beaman arc or Indian pattern clinometer alidade) has a telescope and vertical arc for measuring slopes, used in tachometric plane table surveys for elevation work.
- Historical note: The word “alidade” comes from the Arabic “al-idada” (the revolving ruler). It was used in ancient astronomy on armillary spheres before being adapted for land surveying. The plane table with alidade is one of the oldest field mapping tools still in active use.