Which contour line serves as the main reference in map reading?

The index contour line serves as the main reference in map reading because it is typically marked with bold lines and labeled with elevation values. This makes them easy to identify and distinguishes them from other contour lines on a topographic map. The index contour lines are spaced at regular intervals (often every fifth line), and provide a visual reference for elevation changes across the landscape, allowing map readers to quickly determine the terrain's elevation and to navigate effectively.

In contrast, intermediate contour lines, while they provide more detailed elevation information, do not have elevation labels and are less distinct, making them secondary in the hierarchy of contour lines. Supplementary contour lines are often used in areas of steep terrain or to provide additional detail between index contour lines but are not the primary reference for elevation. Primary contour lines is not a standard term used in map reading; therefore, they do not hold significance in this context.