Chain surveying across an obstruction – effect of raised ground between stations While chaining a line between two stations A and B when a raised ground intervenes between them, which situation typically occurs?

Introduction / Context:
In chain surveying and basic layout, obstacles often interrupt either visibility, chaining, or both. Raised ground (such as a ridge) between two stations is a classic case used to illustrate the choice of indirect methods for alignment and distance transfer. This question checks whether you can correctly diagnose the type of obstruction created by a raised ground between endpoints A and B.

Given Data / Assumptions:

• Stations A and B lie on opposite sides of a ridge or mound.
• Line AB passes over higher intermediate ground.
• We consider direct intervisibility and direct chaining along AB.

Concept / Approach:

With a raised intermediate ground, a straight line of sight from A to B is blocked by the crest; hence intervisibility is lost. Additionally, direct chaining along the straight alignment becomes impractical because of slope, uneven footing, and absence of a straight-line path across the crest at the same elevation. Therefore, both visibility and direct chaining are obstructed, and one must resort to indirect methods (e.g., reciprocal ranging, random line and offsets, or alignment via intermediate points) to establish the distance and direction.

Step-by-Step Solution:

Verification / Alternative check:

Textbook obstacle problems categorize raised ground as causing both types of obstruction in contrast to features like a pond (chaining obstructed, visibility unobstructed) or a building (visibility obstructed, chaining possibly unobstructed around it).

Why Other Options Are Wrong:

Only vision obstructed or only chaining obstructed: incomplete diagnosis for a raised ground crest.

All of the above: includes mutually inclusive but redundant statements; the precise best choice is “both vision and chaining”.

Common Pitfalls:

Assuming you can still range over the crest; forgetting the distinction between visibility-only and chaining-only obstructions used in standard survey problems.

Final Answer:

Both vision and chaining get obstructed