Chain surveying—purpose of tie lines: In a chained framework, tie lines (subsidiary lines) are primarily provided for which purpose?

Introduction / Context:

Chain surveying uses a network of well-conditioned triangles formed by main survey lines. However, interior details (buildings, walls, ponds) often lie away from the main lines. Tie lines are short subsidiary lines introduced to conveniently fix these features without resorting to excessively long offsets that reduce accuracy.

Given Data / Assumptions:

• Plan is controlled by a skeleton of main lines.
• Numerous interior details require measurement from a nearby reference line.
• Offsets should be reasonably short and perpendicular for precision.

Concept / Approach:

The primary purpose of a tie line is to provide an additional base from which offsets for detail can be taken—especially when details cannot be conveniently set out from the main lines. While tie lines can also help check plotting or reduce long offsets, these are secondary benefits; their central role is detail location within the framework.

Step-by-Step Solution:

Verification / Alternative check:

Plotting with tie lines reduces distortion and improves closure of small detail loops; check by resection if available.

Why Other Options Are Wrong:

• Accuracy checks and avoiding long offsets are useful effects but not the principal reason.
• Increasing chain lines indiscriminately complicates the survey with no guaranteed accuracy benefit.
• Reducing sag errors is unrelated; sag is a taping issue, not a tie-line function.

Common Pitfalls:

• Laying a tie line without ensuring it improves offset geometry (perpendicular, short).

Final Answer:

to take offsets for detail survey (locate interior features)