What is defined as the vertical distance between adjacent contour lines?

The correct answer, contour interval, is defined as the vertical distance or difference in elevation between two adjacent contour lines on a topographic map. Contour lines connect points of equal elevation, allowing for the visualization of terrain shapes and slopes. The contour interval is a crucial aspect of interpreting topographic maps, as it provides information on the steepness or gentleness of the terrain; a smaller contour interval indicates a more detailed depiction of slight changes in elevation, while a larger interval may represent more significant changes over a broader area.

In contrast, slope percentage measures the steepness of a slope and is a different concept altogether; it expresses the ratio of vertical rise to horizontal run, often presented as a percentage. Elevation gradient refers to the rate of change in elevation over a certain distance, which can also convey information about terrain steepness but is not the specific measure between contour lines. Depth measurement typically pertains to the vertical distance down from a reference surface, predominantly associated with aquatic environments rather than landform representations. Each of these alternatives addresses different aspects of elevation and terrain analysis, but only contour interval accurately describes the vertical distance between contour lines.