Chain surveying — what is the essential purpose of providing check lines (proof lines) in a chain survey of a small area?

Introduction / Context:
Chain surveying relies on a framework of triangles whose sides are chain lines. Errors can creep in from chaining, ranging, or offsetting. Check (proof) lines provide independent measurements that validate the plotted framework and reveal blunders before plotting is finalized.

Given Data / Assumptions:

• Small, relatively open area suitable for chain surveying.
• Well-conditioned triangles are used as the main framework.
• Check lines can be measured between non-adjacent points.

Concept / Approach:
A check line is measured across a triangle (or between distant points) and compared with its plotted length derived from the main measurements. Agreement within tolerance confirms accuracy; disagreement highlights mistakes in chaining, ranging, or plotting so they can be corrected immediately.

Step-by-Step Solution:

Verification / Alternative check:
Multiple check lines in different parts of the plan ensure robustness; agreement indicates no gross errors remain.

Why Other Options Are Wrong:

• Plotting chain lines/offsets: Aided by field work but not the prime role of check lines.
• Increasing out-turn: Speed is secondary; the principal purpose is quality control.

Common Pitfalls:
Omitting check lines in tight schedules; this often results in distorted plots and costly rework.

Final Answer:
To indicate and verify the accuracy of the survey work (independent check)