Behavior of contours at identical elevation: what is possible? For a normal ground surface represented on a topographic map, two contour lines corresponding to the same elevation may:

Introduction / Context:
Contours are level curves. Their geometry follows from the requirement that each elevation corresponds to a unique locus on a single-valued ground surface (no overhangs). Understanding when same-elevation contours can merge or cross avoids misinterpretation of terrain, especially around ridges, valleys, and flats.

Given Data / Assumptions:

• Ground surface is single-valued with no overhanging cliffs represented.
• Contours are drawn at a fixed interval.
• Standard cartographic conventions apply.

Concept / Approach:

Contours of identical elevation cannot cross, because crossing would imply two different elevations at the same plan location. However, two segments of the same-elevation contour can unite (join) because they are simply different parts of the same closed level curve coming together. On complex terrain, a contour may split and rejoin, but it always remains a single closed line overall (possibly outside the sheet).

Step-by-Step Solution:

Verification / Alternative check:

Examples of ridges or basins show same-elevation contours looping and sometimes rejoining, but never crossing.

Why Other Options Are Wrong:

Crossing (A, D, E) contradicts level-curve properties on a single-valued surface.

C categorically forbids uniting, which is not correct; the same contour can join itself.

Common Pitfalls:

Confusing special cases like overhangs (not shown on standard maps) with general ground; misreading close spacing as an intersection.

Final Answer:

Never cross, but they may unite (join) to form a single contour