What Are the 3 Types of Contour Lines on a Map?

Today, many people use GPS or map applications on their phones to find where to go. However, uses for the topographic map, such as navigation for hiking or mountain climbing, still exist. Understanding what those contour lines mean helps you find your way more easily when trekking out on that next outdoor adventure.

  1. Index Contour Lines

    • Index contour lines stand out the most on a topographical map, as they are the boldest lines on the page. They are designed to stand out like this, as they represent the major intervals of elevation on the map. They form what often look like odd-shaped circles around the map, as these circles follow that particular elevation level. The average topographical map will have its index contour lines spaced out anywhere from 100 to 200 feet in elevation at regular intervals. The actual elevations are often marked along the line at different intervals.

      For example, if an index contour line has the number "100" marked along the circle, this would mean that particular line represents an elevation of 100 feet above sea level, provided the map is using feet as its unit of measure.

    Intermediate Contour Lines

    • Intermediate contour lines are similar to index contour lines. They also form concentric circles at a specific elevation on the map. However, the intermediate contour lines are not marked as boldly on the map, and they represent smaller intervals in elevations.

      For example, if there is an index contour line for each 100 feet of elevation, then intermediate contour lines are represented by four lines in between each contour line, one for each 20 feet of elevation.

      Intermediate contour lines also do not typically have the actual elevation level marked on them, so all you have to do is count the number of intermediate contour lines past the last index contour line and do the math. Using the example, above, if an index contour line was marked as 100 feet, and there are four intermediate contour lines between that line and the next index contour line, you know that each intermediate contour line represents 20 feet of elevation. If the particular area you are viewing on the map is two intermediate lines above the index line, that spot would be at an elevation of 140 feet.

    Supplementary Contour Lines

    • Like index and intermediate contour lines, supplementary contour lines are also concentric circles, but they are depicted on the map as dotted lines. These are only used in areas in which the elevation does not change significantly over a large land area, and are typically simply an even smaller interval of measurement than intermediate contour lines.

      For example, if each intermediate contour line is spaced at an interval of 20 feet, a supplementary contour line might be spaced at every 10 feet of elevation change. However, don't be surprised if you do not see these on your map, since they are only used if the elevation changes very little over a large surface area.