Which of the following statement regarding, prismatic compass survey is correct?

Question

Accepted Answer

Option 3 is correct because in a prismatic compass, the magnetic needle is attached to the graduated circle. This entire assembly (needle and graduated circle) floats freely on a pivot and *does not* move with the box. The box is rotated to align the sights with the object being observed, while the needle and graduated circle remain oriented to magnetic north.

Here's why the other options are incorrect:

*   **Option 1: the magnetic needle moves with the wooden box.** This is incorrect. The key principle of a compass is that the magnetic needle aligns itself with the Earth's magnetic field, independent of the orientation of the box. If the needle moved with the box, it wouldn't be able to provide a bearing.

*   **Option 2: sight vane does not move with the box.** This is incorrect. The sight vane *does* move with the box. The surveyor rotates the box (and therefore the sight vane) to align the sights with the target object.

*   **Option 4: The graduated circle is fixed and the magnetic needle always remain in N-S direction.** This is incorrect. The graduated circle is *not* fixed to the box. It is attached to the magnetic needle and rotates with it. While the magnetic needle does align itself in the N-S direction, the graduated circle rotates along with it to provide the bearing reading relative to the sighting.