A topographic map is a two-dimensional representation of a three-dimensional landscape, illustrating the shape and elevation of the terrain using contour lines. Each contour line connects points of equal elevation above a reference level, typically mean sea level. The contour interval (CI) is the vertical distance or difference in elevation between any two adjacent contour lines on the map. This single, consistent number provides the scale for all vertical measurements, allowing a reader to accurately determine the elevation change across the mapped area. Without knowing the contour interval, the lines are simply abstract shapes, but with it, they translate into quantifiable hills, valleys, and slopes.
Locating the Interval on the Map Legend
The most direct way to find the contour interval is to examine the map’s margin, often referred to as the legend or “collar”. Standardized maps produced by government agencies, such as the United States Geological Survey (USGS), consistently print this information in the map’s lower section. This explicit statement is usually found near the map’s scale bar or other informational text. A common contour interval on a 1:24,000 scale USGS map is 20 feet, and this figure will be clearly stated. Checking the legend first is the quickest method, as it eliminates the need for any calculation. If the number is not immediately visible, however, a simple calculation can be performed using the lines themselves.
Step-by-Step Calculation Using Index Lines
When the contour interval is not explicitly provided in the legend, its value can be determined by focusing on the index contour lines. Index contours are thicker, darker lines that occur at regular intervals, typically every fifth contour line, and are labeled with their specific elevation. These labeled lines serve as the reference points for the entire vertical structure of the map.
The calculation begins by locating two adjacent index contours whose elevation numbers are clearly printed on the map. The first step is to find the difference in elevation between these two labeled lines by subtracting the lower elevation value from the higher one. For instance, if one index line is labeled 7,100 feet and the next is 7,000 feet, the total elevation difference is 100 feet.
The next step involves counting the number of intervals, or spaces, that exist between the two index lines. It is important to count the spaces, not just the unlabeled lines. If there are four unlabeled lines between the two index contours, this creates five total intervals.
To find the contour interval, divide the calculated elevation difference by the total number of intervals counted. Continuing the example, dividing the 100-foot elevation difference by the 5 intervals yields a contour interval of 20 feet. This calculated figure represents the consistent vertical rise or drop between every single contour line on the map.
Practical Application: Reading Elevation and Slope
Once the contour interval is established, it becomes the primary tool for interpreting the terrain’s characteristics and determining the elevation of any unlabeled line. The elevation of any intermediate contour line can be determined by simply adding or subtracting the contour interval from the nearest labeled index line. If the contour interval is 20 feet, and you are moving uphill from a 1,000-foot index line, the next unlabeled line is 1,020 feet, and the one after that is 1,040 feet.
The contour interval is also directly used to assess the steepness, or slope, of the land. The horizontal spacing between the lines indicates the terrain’s gradient. When the lines are drawn very close together, the elevation is changing rapidly over a short horizontal distance, which signifies a steep slope. Conversely, when the contour lines are widely spaced, the elevation is changing gradually, indicating a gentle slope or relatively flat ground.