What are closed contour lines?

Topographic (or contour) maps are the ones with all those weird squiggles and loops. Those strange lines are contour lines, which help show the shape of the land surface. The lines mark elevation — the distance, in feet, above sea level.

All points on a contour line have the same elevation — that is, they're all at the same height above sea level. Slice off the top of a mountain, and the perimeter is a contour line. Move down the mountain and slice off a bit more. That's another contour line.

Every fifth line on a topographic map is thicker and labeled with the elevation. This line is called the index contour; its purpose is to help in counting the elevation on a map.

Contour lines can also show the slope of the land. Here are additional pointers on reading topographical maps:

  • A series of concentric contour lines that are closed onto themselves indicate mountains or hills. Closed contour lines look like misshapen circles or loops. The smallest circle shows the peak.
  • Depression contours show where the land goes into a depression or an indentation on the Earth. These are found in swampy areas or in craters of a mountain. They are also shown with concentric circles; however, these closed contour lines have hachure (tick) marks.
  • The lines get closer to each other as the slope gets steeper.
  • If the area has a gentle gradient, the contour lines are more spread out.
  • If the area is very steep, such as a cliff, the lines may touch, but will never cross over each other.