The darker contour lines on this map are index contours.
A topographic map shows a graphical representation of the shape and location of physical features of land and other physical entities. Military personnel, scientists, hikers, surveyors and others use these maps for varying purposes. A topographic map identifies varying land elevations with contour lines. These lines connect various points of equal elevation with one smooth line. Every point on a contour line has the same elevation. High school earth science students learn to read and interpret topographic maps. One aspect of this process is calculating a contour interval. The contour interval is the difference in elevation between two side-by-side contour lines.
Instructions
1. Examine a topographic map. Look for the index contours. These contour lines have their elevation marked at various points. The line thickness for an index contour is greater than normal adjacent contour lines.
2. Write down the elevations of two consecutive index contours. For example, the index contours may have elevations of 500 feet and 600 feet respectively.
3. Count the number of contour lines between these same two index contours. Do not include the index contours themselves. For example, there may be four contour lines between two index contours.
4. Calculate the contour interval by dividing the change in elevation between the two index contours by the number of contour lines plus one. For example, the difference between 500 feet and 600 feet is 100 feet. If there are 4 contour lines between the indexes, add 1 to reach the number 5. Thus, 100 feet divided by 5 equals 20 feet. The contour interval on this hypothetical topographic map is 20 feet.
Tags: contour lines, index contours, contour interval, contour lines between, feet feet, lines between
