What distinguishes an index contour line from other contour lines on a map?

• A. It is labeled with elevation and typically drawn heavier
• B. It is dashed
• C. It marks a political boundary
• D. It is shown in blue color only

Answer: (A)

Contour lines show equal elevation across the map, and the index contour lines are the bold, labeled ones that mark a specific height. They’re drawn heavier than the surrounding lines and carry the numerical elevation, so you can read the terrain height at a glance. The map usually places these index lines at regular intervals (for example every fifth line), while the lighter, unlabeled lines fill in the elevations between them. This setup lets you quickly judge both the height and the slope: closely spaced lines mean steep terrain, while wider spacing indicates gentler slopes. The other options describe features that aren’t how contour lines are distinguished (dashed lines, political boundaries, or blue coloring).