Setting out circular curves: two-theodolite method When a circular curve is set out by the two-theodolite method, which type of field measurements does the method primarily employ?

Introduction / Context:
Several methods exist for setting out circular curves in the field: deflection angle with chain, long-chord and short-chord methods, tacheometric methods, and the two-theodolite method. Understanding what each method measures helps in choosing the best approach for site constraints (traffic, obstructions, accuracy).

Given Data / Assumptions:

• Two theodolites are set up at the tangent points (or at suitable control points).
• Intersecting lines of sight are used to locate points on the curve.
• Distances along the curve are not chained on the ground.

Concept / Approach:

In the two-theodolite method, one instrument sets the deflection (tangent) direction and the other directs the radial or chord direction so that the intersection of the two lines of sight marks the curve point. Thus, locations are obtained by the intersection of two angular directions without needing linear measurements to each point, which is advantageous where chaining is difficult or unsafe.

Step-by-Step Solution:

Verification / Alternative check:

Classic surveying references explicitly state that the two-theodolite method avoids chaining by relying purely on angle observations and intersection.

Why Other Options Are Wrong:

Linear-only is false; both measurements together are not required here; levelling is unrelated to plan curve setting.

Common Pitfalls:

Attempting to chain along busy roads; the two-theodolite method is designed to eliminate that need.

Final Answer:

Angular measurements only (no chaining to points on curve)