spirit level
alidade
plumbing fork
trough compass
Answer is Wrong!
Answer is Right!
The correct answer is: A. spirit level.
A spirit level is an instrument used to measure and indicate the vertical or horizontal position of a surface. It consists of a glass tube containing a liquid, usually alcohol, with a bubble floating in it. The bubble will always rise to the center of the tube when the surface is level.
A spirit level can be used to center a plane table by placing it on the table and adjusting the legs until the bubble is centered in the tube. This ensures that the plane table is level and that the map will be accurately aligned with the ground.
The other options are incorrect because:
- A alidade is a sighting device used in surveying and navigation. It consists of a straightedge with a telescope or eyepiece at one end and a graduated scale at the other. The alidade is used to measure angles between objects.
- A plumbing fork is a tool used to determine the vertical position of a point. It consists of two legs that are connected at the top by a crosspiece. The legs are adjusted until the crosspiece is horizontal, and the point where the legs meet the ground is the vertical position.
- A trough compass is a type of compass that is used to measure the magnetic declination. It consists of a compass needle that is suspended in a trough of liquid. The needle will always point to the magnetic north pole, and the angle between the needle and the true north pole is the magnetic declination.