Final answer:
Every 5th contour line is darker on a map and is called an index contour line. Index contour lines are essential for reading elevation changes and making sense of the topography on a two-dimensional map.
Step-by-step explanation:
Every 5th contour line on a map is typically darker in color and is known as an index contour line. These index contour lines make it easier to read the elevation on a contour map by acting as references that clearly demarcate the elevation levels at regular intervals. Contour lines are essential in topographic maps as they illustrate the shape and elevation of the land.
Just like hatching is used to create the illusion of shading and form in artwork, including woodblock prints such as The Four Horsemen from The Apocalypse by Albrecht Dürer, contour lines serve to define the shapes and the surface features on a map. They provide the illusion of a three-dimensional landscape on a two-dimensional surface. As you move from one index contour line to the next, you ascend or descend a consistent increment of elevation, making these lines quite valuable for understanding the topography of the mapped area.