How to Determine Elevation on a Contour Map

Contour maps, which are also known as topographic maps or topo maps, are much like other types of maps in that they show the location of pertinent landmarks, such as railroad tracks, roads, rivers, lakes or mountains. Contour maps also show colors that indicate characteristics such as type of vegetation, type of rock formation or bodies of water. Contour maps are different from road maps in that they show an overlay of concentric lines. These are known as contour lines, showing the elevation of land formations above sea level. With a little practice, you can easily read a contour map and determine elevation.

Things You'll Need

  • Topographic or contour map for any area
  • Notepad for jotting notes on location, paths and elevation figures
  • Pencil with eraser
  • Ruler
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Instructions

    • 1

      Examine the margin of the contour map to find the contour interval. The contour interval tells you the distance between adjacent contour lines and is typically printed in the bottom margin. For instance, "CI=10m" means that the contour interval is 10 meters. According to the U.S. Geological Survey, contour intervals typically range from 10 feet in flat areas to 100 feet or more in mountainous regions. The annotation may give two numbers, in which case the first number refers to the general or default interval, while the second refers only to areas of the map where supplementary contour lines appear. For example, you might see an annotation such as "CI=50 ft/supp=10 ft" to indicate a general interval of 50 feet and a supplementary interval of 10 feet.

    • 2

      Find the point on the map for which you wish to determine the elevation. You may want to circle or mark the point so it will be easy to return to later. If the point is on or extremely near to a contour line, take that line as equal to the elevation of the point. If the point is between two lines, the elevation is intermediate between the elevations of those contours. Be aware that the space between contour lines can vary greatly. In flat areas, contour lines are widely spaced. In steep areas, such as cliffs, the contour lines may bunch up or appear to merge into a single thick line. Contour lines never cross, however.

    • 3

      Examine the contour lines in the region of the map in question to determine the direction of elevation change. For instance, if your point is between two contour lines that run roughly east-west, look to the north and south of those lines to find the nearest contour lines in that series that bear numbers. You may find, for example, that elevations are increasing as you go to the south and decreasing as you go to the north -- or vice versa. Typically, every fifth contour line is numbered and appears as a darker or heavier line. These are known as index contours. You may have to trace along the heavy index contour line to find its number.

    • 4

      Examine the contour lines between your point of interest and the nearest index contours. If any of the lines are significantly lighter, they are supplementary contour lines. If any of the lines have hatchmarks along one edge, the contour indicates a decrease in elevation in a series of contours that would otherwise be interpreted as increasing in elevation. For example, a depression or hollow that occurs on top of a hill would show up with a hatched contour line.

    • 5

      Find the elevation of the contour line(s) adjacent to your point of interest by counting up or down from the nearest index contours, using the contour interval. For example, if your point of interest is three lines below an index contour marked 500 and one line above an index contour marked 600, and the contour interval is 20 feet, the point has an elevation intermediate between 560 and 580 feet.