Contour lines are a method of depicting the 3-dimensional character of the terrain on a 2-dimensional map. Just like isobars in the atmosphere depict lines of equal atmospheric pressure, contour lines drawn on the map represent equal points of height above sea level.
Look at the three-dimensional drawing of the mountain below. Imagine that it is an island at low tide. Draw a line all around the island at the low tide level. Three hours later, as the tide has risen, draw another line at the water level and again three hours later. You will have created three contour lines each with a different height above sea level. As you see below, the three dimensional shape of the mountain is mapped by calculating lines of equal elevation all around the mountain, and then transferring these lines onto the map.
On multi-colored maps, contour lines are generally represented in brown. The map legend will indicate the contour interval-the distance in feet (meters, etc.) between each contour line. There will be heavier contour lines every 4th or 5th contour line that are labeled with the height above sea level. The images below illustrate how a variety of surface features can be identified from contour lines.
- Steep slopes - contours are closely spaced
- Gentle slopes - contours are less closely spaced
- Valleys - contours form a V-shape pointing up the hill - these V's are always an indication of a drainage path which could also be a stream or river.
- Ridges - contours form a V-shape pointing down the hill
- Summits - contours forming circles
- Depressions - are indicated by circular contour with lines radiating to the center