Index contour lines are defined as which of the following?

• A. Thick brown lines with elevations marked at frequent intervals
• B. Thin gray lines with no labels
• C. Dotted lines
• D. Red lines with arrows

Answer: (A)

Contour lines on a topographic map show points of equal elevation, and the prominent ones used for quick reference are the index contours. These are drawn as thick brown lines and carry numeric elevation labels at regular intervals, making it easy to read the terrain’s elevation at a glance. The bold index contours are spaced farther apart than the lighter, finer contour lines in between, so you can gauge elevation by looking at both the labeled line and the surrounding line spacing. The thicker, labeled brown lines are specifically designed for fast reading of elevation, which is why that description best fits. Dotted lines, thin unlabeled lines, or red lines with arrows do not match how contour elevation is shown on standard topographic maps.