Setting Out Simple Circular Curves with Two Theodolites – What Is Required? When a simple circular curve is set out using two theodolites from the tangent points, which data are required to turn and lay out the points correctly?

Introduction:
Several methods exist to set out simple circular curves: offsets from tangent/chord, long-chord method, and the theodolite (deflection angle) method. This question assesses whether you know what specific quantities are needed when two theodolites are used at the tangent points to turn points on the curve.

Given Data / Assumptions:

• Two theodolites are stationed at tangent points (T1 and T2).
• Curve parameters (radius, deflection angle, chord length) are known.
• The method relies on angular setting rather than transverse offsets.

Concept / Approach:

In Rankine’s method, the curve is set out by turning successive deflection angles from the tangent at the point of commencement. Deflection angles to each chord (or sub-chord) are computed and turned at the instrument, placing pegs at the chord ends to trace the curve. Using two theodolites allows work from both ends, meeting in the middle.

Step-by-Step Solution:

Verification / Alternative check:

Survey texts explicitly name this as theodolite (deflection angle) method, contrasting it with offset-based techniques.

Why Other Options Are Wrong:

Offsets from tangent/chord/long chord refer to different field procedures; they are not the data used in the two-theodolite deflection method.

Common Pitfalls:

Mixing formulas and steps from different curve-setting methods; forgetting to apply sub-chord corrections near the tangency points.

Final Answer:

Deflection angles computed by Rankine's method