Contours and terrain interpretation: Contour lines of different elevations can meet and merge into a single line only under which special terrain condition?

Introduction / Context:
Contours are the backbone of topographic interpretation in civil engineering. They represent lines of equal elevation and normally do not cross or unite. Recognizing the single exception helps avoid mapping mistakes and improves terrain reading for route location, drainage, and site planning.

Given Data / Assumptions:

• Contours are drawn at uniform vertical intervals (e.g., 1 m, 5 m).
• Each contour corresponds to a unique elevation.
• We consider ideal, correctly drafted sheets without drafting errors.

Concept / Approach:
Different elevation contours cannot meet because a point cannot simultaneously have two elevations. However, at a near-vertical or truly vertical face, the horizontal distance between successive elevation levels approaches zero. On the map, successive contours then coincide, appearing to merge into one line. This is the only legitimate case of contour “meeting.” Saddles, ridges, and hilltops show contours that approach, loop, or form characteristic shapes but do not unite into a single line.

Step-by-Step Solution:

Verification / Alternative check:
Field checks on cliff faces show nearly vertical rock walls where plan distances between elevation bands are negligible, matching the map behavior.

Why Other Options Are Wrong:

• Saddle: contours pinch but remain distinct.
• Watershed line: contours form V-patterns but do not merge.
• Hill top: contours close into separate, nested loops with decreasing area.

Common Pitfalls:
Misreading tightly spaced contours as crossing; mislabeling depressions as hilltops; ignoring index contour elevation labels.

Final Answer:
A vertical cliff (nearly vertical face)