Optical-reading theodolites – behavior of vertical circle graduations In optical-reading instruments, which of the following statements about the vertical circle readings is generally correct?

Introduction / Context:
Optical-reading theodolites and total stations have vertical circles whose graduations and zero settings are defined by instrument design. Understanding how the reading changes with telescope elevation for each face is important for correct booking and for applying face-left/face-right procedures that reduce systematic errors. This question reviews standard conventions found in many optical-reading instruments.

Given Data / Assumptions:

• Instrument uses a continuous vertical circle (0°–359°) rather than quadrantal indexing.
• Face-left and face-right refer to standard positioning of the telescope and vertical circle relative to the observer.
• We assume the common convention used by many instruments and textbooks.

Concept / Approach:

Many optical-reading instruments adopt a continuous vertical circle to avoid ambiguity in elevation/zenith angles. With a particular zero reference and circle orientation, elevating the telescope increases the readout in face-left, while the same elevation produces a decreasing readout in face-right (because the circle is viewed from the opposite side and the index orientation reverses). These trends underpin booking rules that ensure consistency across faces and enable averaging to eliminate certain instrumental errors.

Step-by-Step Solution:

Verification / Alternative check:

Instrument manuals and surveying texts present tables showing opposite-sense changes in vertical readings between faces, confirming the described increase/decrease rules and continuous-circle practice.

Why Other Options Are Wrong:

Each individual statement is correct under the stated convention, so the combined option is the most accurate.

Common Pitfalls:

Assuming every brand uses identical zero references; forgetting to note face position during booking; mixing elevation angle with zenith angle definitions.

Final Answer:

All of the above