Topographic maps are specialized tools that translate the three-dimensional shape of the Earth’s surface into a two-dimensional visual representation. These maps use lines to illustrate the height and form of mountains, hills, and valleys, providing a powerful way to visualize terrain. The contour interval is the single most important piece of information for interpreting the elevation changes shown on these maps. This value allows anyone to understand the landscape’s vertical profile as depicted on the map.
What Defines a Contour Line
A contour line is a continuous line drawn on a topographic map that connects all points of equal elevation above a specific reference datum, which is typically mean sea level. If a person were to walk along the path of a single contour line, they would remain at the exact same height throughout their journey. These lines are the fundamental building blocks used to illustrate the topography, or relief, of the mapped area.
Contour lines appear in two main forms to aid readability across the map. Most are standard contour lines, which are thin and usually unlabeled with a specific elevation number. To prevent the map from becoming overly cluttered, every fifth line is represented by a thicker, darker line known as an index contour. These index lines are labeled with their corresponding elevation, providing clear markers for quick reference across the terrain.
The Standard Interval Measurement
The contour interval (CI) is defined as the fixed, consistent vertical distance, or difference in elevation, between any two adjacent contour lines on a map. This difference is constant across the entire map, ensuring that the rate of vertical change is uniformly represented everywhere. For example, if a map has a contour interval of 20 feet, moving from one line to the very next line means the elevation has either increased or decreased by exactly 20 feet.
The value for the contour interval is a mandatory piece of information and is usually found printed in the map’s legend or marginal information. If the interval is not explicitly stated, it can be calculated by using the labeled index contours.
Calculating the CI
To determine the CI, one must first find the difference in elevation between two adjacent index lines. This difference is then divided by the number of intervals, which is the total count of lines between the two index contours, including the unlabeled ones. For instance, if two index contours are labeled 500 feet and 600 feet, and there are four thinner lines between them, the calculation involves dividing the 100-foot difference by five. This calculation reveals a contour interval of 20 feet. By knowing this constant vertical distance, the elevation of every unlabeled line on the map can be determined.
Interpreting Terrain Steepness
The contour interval is the basis for interpreting the steepness, or slope, of the terrain by examining the horizontal spacing between the lines. While the vertical difference between any two adjacent lines is always the same (the CI), the horizontal distance covered to achieve that vertical change varies. This horizontal distance is the visual clue used to assess the steepness of the ground.
When contour lines are drawn close together, they indicate a steep slope, meaning a rapid change in elevation occurs over a short horizontal distance. Conversely, when the lines are spaced far apart, they represent a gentle or gradual slope. Uniformly spaced contour lines across a section of the map suggest a relatively constant slope throughout that area. Other features, like hachure marks—small tick marks pointing inward—on a line of concentric circles indicate a depression or a sunken area.