Contours and terminology: The constant vertical distance between two adjacent contour lines on a map is known as the ______.

Introduction / Context:
Contours are lines joining points of equal elevation on a map. The spacing between successive contours represents slope, while the nominal vertical difference between them is a key map parameter used in design and volume computations. Correct terminology prevents confusion in communication and calculations.

Given Data / Assumptions:

• Contours are at a fixed elevation step across the map.
• The map specifies a single value for the vertical step (e.g., 1 m, 5 m, 10 m).
• No variable-interval contouring is considered.

Concept / Approach:

The constant vertical difference between two successive contours is the contour interval. By contrast, the horizontal spacing between the same two contours on the ground (or map) depends on the ground slope and is sometimes called the horizontal equivalent or horizontal interval, which is not constant over the sheet unless the slope is uniform.

Step-by-Step Solution:

Verification / Alternative check:

On any standard topographic map, the legend lists the “contour interval” as the primary parameter describing vertical resolution.

Why Other Options Are Wrong:

Horizontal interval/equivalent refer to ground or map distance between contours and vary with slope; vertical equivalent is a nonstandard term; contour gradient is a line of constant slope, not the vertical spacing of contours.

Common Pitfalls:

Assuming wide spacing always means flat ground without checking the legend; confusing interval (vertical) with spacing (horizontal).

Final Answer:

Contour interval