__________ Instrument is used to sight to an object.

Question

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Option D: Alidade

Explanation & Concepts:
An alidade is a sighting instrument used in plane table surveying to draw rays toward an object or station. It consists of a straight edge and a sighting mechanism (either simple vanes or a telescope) to establish the line of sight.

Working Principle: The alidade is placed on the drawing board and rotated until the target object is visible through the sights. Once aligned, a pencil line is drawn along the fiducial edge of the alidade onto the drawing sheet, representing the direction of the object from the station point.

Key Formulas: $L = D 	an(\alpha)$ — where $L$ is the height difference, $D$ is the horizontal distance, and $\alpha$ is the vertical angle measured by a telescopic alidade.

Key Points: Simple alidade uses two vertical vanes (object vane and sight vane) to define the line of sight.; Telescopic alidade is used for higher precision and measurement of vertical angles.; The fiducial edge of the alidade is always kept in contact with the station point marked on the drawing sheet.

Advantages: Allows direct plotting of rays in the field; Simplifies graphical triangulation

Disadvantages: Simple alidade is limited to short-range sighting; Requires stable table mounting to avoid errors

Applications: Plane table surveying for topography mapping; Setting out points from a base line

Additional Info: Spirit level is used for leveling the plane table (ensuring it is horizontal).. Trough compass is used for orienting the plane table with respect to magnetic north.. U-fork with plumb bob is used for centering the plane table over the ground station.

Answer: D is correct — The alidade is the specific instrument used to define the line of sight when plotting directions on a plane table.

Exam Tip: In surveying exams, remember that the Alidade performs the 'sighting' (direction), whereas the Trough Compass provides the 'orientation' (bearing).

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Option A: Trough compass

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Option B: Spirit level

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Option C: U-fork with plumb bob

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