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Contour Interval of the MAP

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A contour line is a (usually brown) line on a map joining points of equal altitude (height above mean sea level).

The contour interval is the distance vertically between the altitude each individual contour line represents. When reading a map, contour lines give you information on what the topography of the area is likely to be. The closer the contours the steeper the ground. By interpreting contour lines it is possible to imagine a 3 dimensional picture of what the ground might look like.

Some 1:50,000 scale maps of the Rocky Mountains have a contour interval of 100 feet; i.e. separate contour lines are drawn joining points of altitude 4000, 4100, 4200, 4300 etc feet above mean sea level. 1:250,000 maps of the area have a contour interval of 500 feet. Using a 100 foot interval on these maps of the Rockies would make them too cluttered. There are some 1:50,000 scale maps of the Rockies that have a 20 meter contour interval below 2000 meters and a 40 meter interval above 2000 meters - these can be very confusing. Others have a 50 meter contour interval. So it is important to check the contour interval on every map! However in Saskatchewan some 1:50,000 scale maps have a contour interval of 10 feet and 1:250,000 scale maps have a contour interval of 100 feet. This is necessary to show any variation of altitude there. In the mountainous areas of the US, contour intervals seem to be 80 feet on the 1:62500 scale maps and 40 feet on the 1:40680 scale and 200 feet on the 1:250,000 maps.

Understanding Contour Lines

At first glance all the wiggly brown lines on a map may seem meaningless but detailed study of them can reveal a lot about the landscape. With practice, from studying contour lines you will be able to form a 3-dimensional picture in your mind of what an area may look like.
  • The closer the contour lines the steeper the ground.
  • Closed contour lines represent mountains or hills.

    hill

  • Closed contour lines with dashes represent depressions or hollows.

    depression

What about other shapes of contours that represent ridges and valleys? Which are the ridges and which are the valleys? The analysis is often made easier by there being a blue line representing a stream, and streams usually run in valleys. A good way to find out where the ridges and valleys are is to Draw a Section:

map section

One must realize that this is an approximation of the terrain. The following illustrates two equally valid interpretations of contours:

two valid interpretations

two valid interpretations

two valid interpretations

With continued study of topographic maps and contour lines you will be able to picture your hike from the map while sitting in your armchair. However that must not stop you getting out there and doing it!!

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