Topographic surveying—definitions: An imaginary line that joins points on the Earth's surface having exactly the same elevation (reduced level) is called what?

Introduction / Context:

Contour mapping is central to civil engineering design for roads, canals, embankments, and drainage. Understanding the precise meaning of contour-related terms helps in interpreting maps and preparing longitudinal and cross sections for earthwork computations.

Given Data / Assumptions:

• The question asks specifically for the term describing a line joining points of equal elevation (reduced level) on a plan or map.
• Standard plane surveying conventions apply.

Concept / Approach:

A contour line is the locus of points with the same elevation relative to a chosen datum (e.g., mean sea level). On maps, these lines depict terrain relief; close spacing means steep slopes, while wide spacing indicates gentle slopes. By contrast, a contour surface is the three-dimensional surface itself; a contour gradient is a line with a constant slope on the ground; and a level line is a line everywhere perpendicular to the direction of gravity (equipotential) and, strictly speaking, curved with the Earth.

Step-by-Step Solution:

Verification / Alternative check:

Any standard topographic sheet shows multiple contour lines labeled by elevations; interpolating between them estimates ground levels and slopes.

Why Other Options Are Wrong:

• Contour surface: 3D surface, not a map line.
• Contour gradient: constant-slope line on ground between two contours.
• Level line: line of equal potential, not a mapping locus of equal elevation points across varying plan positions.
• None: incorrect because the proper term exists.

Common Pitfalls:

• Confusing level lines (which are truly horizontal) with contour lines (which follow the ground on a plan).

Final Answer:

contour line