Contours in landform drawings: A “contour interval” is defined in surveying/cartography. Is it the vertical difference in elevation between successive contours, or the horizontal distance between them?

Introduction / Context:
Understanding contour terminology is fundamental to reading landform drawings, topographic maps, and civil site plans. This item distinguishes the contour interval from planimetric spacing on the sheet, which varies with terrain steepness and map scale.

Given Data / Assumptions:

• Contours are lines connecting points of equal elevation.
• Successive contours are adjacent lines representing elevations separated by a fixed step.
• The map has a stated contour interval in units (e.g., 1 m, 5 ft).

Concept / Approach:
The contour interval is defined as a constant vertical increment in elevation between adjacent contour lines. Horizontal spacing on the paper is not fixed; it tightens in steep terrain and spreads out on gentle slopes. Therefore, any definition tying the interval to horizontal distance is incorrect.

Step-by-Step Solution:

Verification / Alternative check:
On a hillside, the physical horizontal distance between contours changes with slope. If interval were horizontal, it would be constant on the sheet, which it is not. The constant dimension is elevation step, not plan distance.

Why Other Options Are Wrong:

• Horizontal distance (option A) confuses spacing with interval.
• Slope percent (option C) is derived, not the definition.
• Map scale (option D) relates drawing to real-world distances, not elevation steps.
• Azimuth change (option E) concerns direction, not elevation.

Common Pitfalls:
Assuming equal spacing of contours on paper equals equal ground distance; forgetting that closely spaced contours indicate steep slopes.

Final Answer:
Correct: it is the vertical elevation difference between successive contours