Framework checks in chain surveying: What are the additional lines measured specifically to check the accuracy and correctness of the chain survey framework called?

Introduction / Context:
Chain surveying relies on a network of triangles built on well-chosen base lines. To guard against accumulation of small errors, extra measurements are added to verify the plotted framework before committing to final plans.

Given Data / Assumptions:

• Primary framework consists of main and subsidiary triangles.
• Some independent measurements are taken besides the essential sides.
• Purpose is verification, not locating new details.

Concept / Approach:
Additional lines measured to verify the framework are called check lines (often also termed proof lines). Their plotted lengths should agree with measured values within tolerance; disagreement signals blunders or poor triangle shape. Tie lines, in contrast, connect interior points to the framework for detail location, not for checking accuracy.

Step-by-Step Solution:

Verification / Alternative check:
Multiple non-collinear check lines provide redundancy; agreement among them increases confidence in the plotted framework.

Why Other Options Are Wrong:

• Tie lines locate details and do not serve as independent accuracy checks.
• Base lines and offset lines have different roles and are not primarily for verification.

Common Pitfalls:
Calling any extra line a “check line”; using a very short or nearly collinear check line that has little diagnostic power.

Final Answer:
Check lines (also called proof lines)