What is the purpose of supplementary contour lines on a topo map?

• A. They indicate index contours
• B. They indicate elevation change is minimal and are dotted lines added when there is a large space between index and intermediate contours
• C. They mark depressions
• D. They mark water features

Answer: (B)

Supplementary contour lines provide extra elevation detail where the usual contour lines are spaced far apart. When the terrain is flat or gently sloping, the main contours can be far apart, making it hard to read subtle changes in elevation. Adding these dotted, lighter lines between the major contours fills in that gap, showing how elevation changes at intermediate steps and giving a clearer picture of the landform.

They aren’t the bold, labeled index contours, and they don’t mark depressions or water features. Their job is simply to add resolution between the major lines to reveal gentle slopes and small elevation changes.