Topographic maps have many features that make it hard for people to understand the map if they don’t know what those features are. Perhaps the most important features of topographic maps are contour lines. Contour lines are brown lines that show terrain, elevation, and slope. There are three types of contour lines, Index, Intermediate, and Supplementary. Index lines are thick lines with numbers, representing the number of feet the area is above sea level. Intermediate lines are lines that are in between index lines. These two lines are used to find the Contour Interval, or the amount of feet that increases or decreases for each contour line.
To find the contour interval, count the amount of intermediate lines in between two index lines. Take the two numbers on the index lines and subtract the lesser number from the greater number. Divide that by five and end up with the contour interval. The contour interval can also be found at the bottom margin. The common contour interval is twenty feet, but mountainous areas may have one-hundred feet or more as the interval.
In addition, contour lines display many geographic features. Hills, for example, are shown by irregular circles around each other. The smallest circle is the top of the hill. Saddles are low areas between two hills or mountains. Saddles are shown somewhat like a rectangle. There are two hills across from or next to one another and a few spaced lines on the other side, which aren’t as steep as the hill.
Valleys are simply shown as repeated v-like contour lines, sometimes with a river flowing through the middle of them. Ridges are shown by a hill with circles that get close together on two sides and space out on at least one other side. Cliffs are shown by lines that touch or almost touch. Hills, Saddles, Cliffs, Ridges, and Valleys are the basic features that most topographic maps have and how to identify them when observing a topographic map.