Tacheometry — why is vertical staff holding generally preferred over normal (perpendicular) staffing during tacheometric observations? Choose the most appropriate reason.

Introduction / Context:
Tacheometry (stadia surveying) computes distances and elevations from staff intercepts and vertical angles observed through a tacheometer. The way the staff is held affects the intercept and thus the derived quantities. Practitioners often prefer a particular holding because it reduces sensitivity to small errors in staff orientation.

Given Data / Assumptions:

• Stadia readings are taken on a graduated staff.
• Observer measures upper and lower stadia hair coincidences and the vertical angle.
• Choice between vertical staff holding and “normal” staffing (perpendicular to the line of sight).

Concept / Approach:
With the staff held truly vertical, small deviations by the staffman cause smaller proportional errors in the stadia intercept than when trying to hold the staff normal to the oblique line of sight, which is harder to judge. Thus, vertical staff holding minimizes the effect of careless holding and simplifies routine instructions to the staffman, improving reliability of results.

Step-by-Step Solution:

Verification / Alternative check:
Field practice and textbooks recommend vertical staffing for routine tacheometry to control human-factor errors in staff orientation.

Why Other Options Are Wrong:

• Ease of reduction: Possible advantage, but the principal benefit is error minimization.
• Facility of holding: Vertical is simple, but the core reason is reduced sensitivity to tilt.
• None: Incorrect because a strong reason exists.

Common Pitfalls:
Instructing staff to be “normal to line of sight,” which is difficult to judge and leads to larger, inconsistent errors.

Final Answer:
Minimum effect of careless holding on the computed result