Understanding the Alidade in Surveying

The question asks about the definition of an alidade. An alidade is a crucial instrument used specifically in a surveying technique called plane table surveying.

Alidade Function in Plane Table Surveying

In plane table surveying, a drawing sheet is fixed onto a table (the plane table) mounted on a tripod. The surveyor uses the alidade to sight objects or points in the field and simultaneously draw lines on the drawing sheet representing the direction to those points from the current station.

• The alidade essentially consists of a rule or straight edge with some form of sighting device.
• The sighting device allows the surveyor to accurately aim at the target point.
• The straight edge is then used to draw a line on the paper along the direction of sight, passing through the station point plotted on the paper.

This process helps in plotting the positions of different objects and creating a map directly in the field.

Evaluating the Options

Let's look at the given options in the context of what an alidade does:

1. An instrument used to hold the chain in place during the survey: This is incorrect. Chains are used for measuring distances, and they might be held by pins or arrows, not an alidade.
2. An instrument used to measure vertical angles: This is incorrect. Instruments like a theodolite or a clinometer are used for measuring vertical angles.
3. An instrument used for sighting the point in plane table surveying: This statement accurately describes the primary function of an alidade in plane table surveying. It is used to sight distant objects and draw lines indicating their direction.
4. An instrument used to measure distances: This is incorrect. Instruments like chains, tapes, or electronic distance measurement (EDM) devices are used for measuring distances.

Based on the function of the alidade in plane table surveying, the correct description is that it is used for sighting points.