A contour interval is the fixed vertical distance separating one contour line from the next on a topographic map. This interval quantifies the change in elevation across a given area. Understanding this value is necessary to accurately gauge the steepness of slopes and the overall shape of geographic features. The following steps provide the necessary methods to locate and calculate this measurement.
The Role of Index and Intermediate Contours
Topographic maps use two distinct types of lines to represent elevation change. Index contours are the heavier, thicker lines that appear at regular intervals and typically have their elevation value printed directly on them. These labeled lines serve as reference points, providing specific, known altitudes above sea level. They usually occur every fifth contour line to make reading the map easier.
The lines that lie between these labeled boundaries are known as intermediate contours. These are thinner and are not individually labeled with an elevation value. Intermediate contours show the gradual changes in height that occur between the major index lines.
The difference in elevation between any two adjacent intermediate lines is the contour interval itself. The standardized spacing of these two line types creates a reliable grid used as the basis for all elevation calculations.
Calculating the Contour Interval
The systematic process for finding the contour interval relies on the information provided by the index lines. Begin by locating any two adjacent index contours on the map that have their elevation values clearly printed. For instance, one index line might be labeled 500 feet, and the next heavier line may be labeled 600 feet. These two numbers establish the total difference in elevation across the segment being analyzed.
The next step involves calculating the total vertical separation between these two reference points. Subtract the lower elevation from the higher elevation to find the total rise over that section of the map. In the example of 500 feet and 600 feet, the difference is 100 feet. This difference is distributed evenly across all the spaces between the two index lines.
After determining the elevation difference, the next step is to count the number of spaces or intervals that exist between the two index contours. It is important to count the spaces and not the individual intermediate lines themselves. The standard convention for most topographic maps is to place four intermediate lines between two index lines, which creates five distinct intervals.
The final step is a simple division that yields the contour interval. Divide the total difference in elevation (e.g., 100 feet) by the number of intervals counted (e.g., 5 spaces). Using the example, 100 feet divided by 5 equals 20 feet. This result, 20 feet, is the contour interval, meaning the vertical separation between any two adjacent lines on the entire map is 20 feet.
If the index lines are, for example, 1,000 meters and 1,250 meters, the elevation difference is 250 meters. If there are four intermediate lines, yielding five spaces, the calculation would be 250 divided by 5, resulting in a 50-meter contour interval.
How Map Scale and Terrain Affect the Interval
The selection of the contour interval is dictated by the characteristics of the terrain and the map’s scale. For areas featuring very steep slopes, a larger interval (e.g., 40 or 80 feet) is chosen. This prevents the contour lines from merging into an unreadable block of ink.
Conversely, relatively flat terrain requires a smaller contour interval (e.g., 10 or 20 feet). A smaller interval ensures that subtle changes in elevation are captured, providing necessary detail that would be lost with a large interval.
The map’s scale also influences the chosen interval. Maps with a larger scale, which show a smaller geographic area in high detail, utilize smaller intervals to maintain precision. Maps with a smaller scale, which cover vast regions, must use larger intervals to avoid excessive line density.