On a topographic map what indicates the difference in elevation between adjacent lines

The Contour Intervals (CI)

Explanation:

In topographic maps contours represent an Isoline of elevation data, but because there can be many contour lines in a given map and they would densify the information represented not every contour line is labeled with their elevation. Index Contour lines are the ones that are labeled the other contour lines are still drawn but not labeled therefore to know the distances between adjacent contour lines and not just index contour lines the topographer indicates in the maps through Contour Intervals.

Contour intervals can be easily calculated by getting the difference between two adjacent Index Contour lines and then dividing by the number o contours between both index lines plus 1.

Example

CI= (1200m - 1000m) / 4 + 1 = 200m / 5

CI= 40m