A contour interval defines the vertical distance between successive contour lines on a topographic map. Understanding this measurement is essential for accurately interpreting the terrain. It provides a standardized way to visualize the three-dimensional landscape in a two-dimensional format, helping map users comprehend the steepness of slopes and the overall relief of an area.
The Basics of Contour Lines
Contour lines are the foundational framework of topographic maps, connecting points of identical elevation above a specific reference level, such as sea level. These lines illustrate the shape and elevation of the landscape, including features like hills, valleys, and mountains. Following a single contour line in real life would mean traversing a path without any change in vertical height.
Contour lines exhibit several consistent characteristics that aid in map interpretation. They never cross one another, as a single point cannot simultaneously exist at two different elevations. While they may appear very close together in areas of extreme steepness, they maintain their distinctness. All contour lines eventually form closed loops, even if these closures extend beyond the boundaries of the map. The spacing between these lines directly indicates the slope of the terrain; closely spaced lines signify a steep incline, while widely spaced lines suggest a gentle slope or relatively flat ground.
Identifying the Contour Interval
The contour interval is typically provided directly on the map, often in the legend or margin. This numerical value explicitly tells you the consistent vertical difference in elevation between any two adjacent contour lines.
For instance, if a map indicates a contour interval of 20 feet, it means that each contour line represents a 20-foot elevation change from the line next to it. To determine the elevation of an unlabeled contour line, you can identify a nearby labeled line and then count the number of intervals, adding or subtracting the contour interval value accordingly. If the map does not explicitly state the contour interval, it can be calculated by finding two labeled contour lines, subtracting the lower elevation from the higher, and then dividing that difference by the number of contour lines (or spaces) between them. This method allows for elevation estimation.
What the Contour Interval Reveals
A small contour interval, such as 10 or 20 feet, indicates a map designed for areas with gentle slopes or relatively flat topography. This smaller interval allows for the depiction of subtle elevation changes, providing a highly detailed representation of the land’s surface. Such maps are useful for activities requiring precise elevation data, like detailed hiking route planning or engineering projects in areas with minimal grade.
A large contour interval, perhaps 50 or 100 feet, is typically used for maps covering rugged or mountainous terrain. In these environments, large elevation changes occur over short horizontal distances, and a smaller interval would result in an overcrowded and illegible map. While a larger interval provides a broader overview of the landscape, it offers less detail about minor terrain features. These maps are suitable for general navigation in areas with dramatic elevation shifts, where the focus is on major topographical elements rather than minute variations.