Prismatic compass — where is zero (0°) placed on the graduated circle?
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AAt the north end
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BAt the east end
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CAt the south end
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DAt the west end
Answer
Correct Answer: At the south end
Explanation
Introduction / Context:Unlike surveyor’s compasses, the prismatic compass has a distinctive graduation system used to read bearings directly while sighting. Recognizing the position of zero and the clockwise graduation order helps avoid 90° misreads and ensures correct plotting.
Given Data / Assumptions:
- Standard prismatic compass graduation is clockwise.
- Cardinal points correspond to specific degree values.
Concept / Approach:In a prismatic compass, 0° is at the south end, 90° at the west, 180° at the north, and 270° at the east. Readings increase clockwise. This arrangement allows direct forward bearings when sighting through the prism while aligning the line of sight with the object.
Step-by-Step Solution:
Recall the standard layout: S = 0°, W = 90°, N = 180°, E = 270°.Confirm clockwise graduation.Therefore, zero is at the south end.Verification / Alternative check:Most instrument dials and textbooks illustrate the prismatic compass with this graduation; a quick look at a dial confirms the S-0° marking.
Why Other Options Are Wrong:
- North/east/west ends do not carry 0° on the prismatic compass; they are at 180°, 270°, and 90° respectively.
Common Pitfalls:Confusing prismatic with surveyor’s compass conventions; always check the instrument type before converting bearings.
Final Answer:At the south end