Topographic interpretation rule: Contours of different elevations can cross each other only under which special terrain condition?

Introduction / Context:
Contours represent loci of equal elevation and ordinarily never cross or merge. Recognizing the few exceptional cases prevents serious drafting and interpretation mistakes in topographic surveying and civil site planning.

Given Data / Assumptions:

• Contours are drawn at uniform vertical intervals.
• Each contour corresponds to a unique elevation value.
• We consider accurate, properly drafted maps.

Concept / Approach:
Contours cross only when the surface above a given plan location is present at two or more elevations vertically above the same plan point. This happens with an overhanging cliff (undercut face), where higher rock strata project over lower ground. On a vertical cliff, contours become coincident (touch) but do not cross; on saddles and inclined planes they remain separate and ordered.

Step-by-Step Solution:

Verification / Alternative check:
Field sketches of caves or overhangs confirm that the plan projection overlaps different elevation edges, matching the crossing appearance on contour maps.

Why Other Options Are Wrong:

• Vertical cliff: contours coincide (merge) because horizontal spacing is zero, but they do not cross.
• Saddle and inclined plane: contours remain distinct and ordered; no crossing occurs.

Common Pitfalls:
Confusing “coinciding contours” on a vertical cliff with “crossing contours”; misreading dense contour spacing as overlap; ignoring elevation labels.

Final Answer:
An overhanging cliff (undercut face)