Under what condition are supplementary contour lines added to a map?

• A. When there is little elevation change
• B. When index contours are very close together
• C. When there is a lot of space between index and intermediate contours
• D. When there are no data points

Answer: (C)

Supplementary contour lines are added to increase contour density where the elevation changes are gradual and the gap between the heavier index contours and the lighter regular contours would otherwise be large. When there’s a lot of space between index contours and the intermediate lines, extra lines help show subtle rises and falls in terrain, making the map easier to read and interpret. So, the condition described is precisely that large spacing between index and intermediate contours. If elevation changes are already well-represented by existing lines (or if there are no data points to base additional lines on), supplementary lines aren’t used.