What is a contour line and what are its characteristics?

A contour line is an imaginary line connecting points of equal elevation on a map. Characteristics include: contour lines are continuous and never cross each other (except for overhanging cliff), closely spaced contours indicate steep slope while widely spaced indicate gentle slope, contours form closed loops around hills or depressions, and the contour interval (vertical distance between successive contours) is constant for a map. Contour maps are essential for understanding terrain and designing infrastructure.