Compass instruments – why a prismatic compass gives better reading accuracy A prismatic compass is generally considered more accurate than a surveyor’s compass primarily because:

Introduction / Context:
Both prismatic and surveyor’s compasses are used to measure magnetic bearings in the field. However, practical accuracy differs due to mechanical design and how readings are observed. This question asks you to identify the major design features that give the prismatic compass an edge in typical surveying practice.

Given Data / Assumptions:

• We compare reading convenience and reduction of parallax and estimation errors.
• We focus on scale type and viewing method.
• Magnetic environment and needle quality are assumed comparable.

Concept / Approach:

The prismatic compass uses a prism to magnify and bring the graduated circle into focus at the same time as the target is sighted, allowing the observer to read bearings directly without shifting eye position. It also uses whole-circle bearings (0°–360°), which reduce confusion between quadrants and often minimize transcription errors. These features together improve reading speed and accuracy compared with the surveyor’s compass that typically uses quadrantal bearings and separate viewing of sights and graduations.

Step-by-Step Solution:

Verification / Alternative check:

Standard surveying manuals emphasize WCB scales and prismatic reading as advantages of the prismatic compass, corroborating the selection of both (c) and (d).

Why Other Options Are Wrong:

Better needle: not the main differentiator; both instruments use adequate needles.

Sliding glass: a protective feature, not a primary accuracy enhancer.

Common Pitfalls:

Confusing magnetic accuracy with reading accuracy; attributing improvements to needle magnetization instead of optical reading and scale system.

Final Answer:

Both (c) and (d)