A topographic map is a two-dimensional representation of the Earth’s three-dimensional surface, using specialized lines to illustrate the shape and elevation of the land. These curved lines, known as contour lines, connect all points on the map that share the exact same elevation above a specific reference point, typically mean sea level. The crucial measurement that allows a map reader to translate these lines into tangible terrain is the contour interval. This fixed value represents the consistent vertical difference between one contour line and the next, providing the fundamental key to decoding the landscape’s elevation changes. Understanding this single measurement transforms a flat piece of paper into a detailed model of hills, valleys, and slopes.
Defining the Vertical Measurement
The contour interval is the constant vertical distance separating any two adjacent contour lines on a map. If a map states an interval of 20 feet, moving from one line to the next means ascending or descending exactly 20 feet in elevation. This value is determined by the mapmaker based on the terrain’s ruggedness and the map’s overall scale, and it is always stated clearly in the map’s legend.
For instance, a large-scale map (such as 1:12,000) covering a small area can use a smaller interval like 10 feet. Conversely, a small-scale map covering a vast, mountainous region might utilize a larger interval, perhaps 80 or 100 feet, to prevent the map from becoming cluttered.
To simplify reading, cartographers employ two types of contour lines. Index contours are drawn with a thicker line and are labeled periodically with their exact elevation number. These index contours typically occur every fifth line, making it easy to quickly determine elevation. The lines falling between the index contours are called intermediate contours. These are thinner, unlabeled lines, and their elevation is determined by counting up or down from the nearest index contour using the map’s specified contour interval.
Interpreting Terrain Steepness
The most practical application of the contour interval is determining the gradient or steepness of the terrain. While the interval is a vertical measure, the horizontal spacing between the lines provides the necessary information to calculate the slope.
Closely spaced contour lines indicate a steep slope, where the elevation changes rapidly over a short horizontal distance. In contrast, contour lines that are spread far apart represent a gentle or gradual slope. In this scenario, the fixed vertical elevation change is distributed across a much greater horizontal distance. Observing the variations in spacing across a map allows a reader to visualize the shape of a landform, identifying cliffs, rolling hills, or flat plateaus.
A uniform distance between lines indicates a constant slope, meaning the angle of the incline does not change. If the spacing increases toward the top of a hill, it suggests the hill’s base is steeper than its summit. This direct relationship between line spacing and slope allows hikers and engineers to plan routes and construction projects based on the map’s representation of the terrain.
Essential Rules of Contour Lines
Contour lines adhere to universal rules that govern their configuration and ensure the accurate depiction of topography.
A fundamental principle is that contour lines can never cross one another, because a single point on the Earth’s surface cannot exist at two different elevations simultaneously. The only exception is a sheer vertical cliff or an overhang, where the lines may appear to merge or touch briefly.
When lines form a closed loop, they indicate a landform with a summit, such as a hill or a mountain peak. If the closed loop has small tick marks, known as hachure marks, pointing inward, it signifies a depression or a basin, indicating that the elevation decreases toward the center.
When a contour line crosses a stream or a drainage area, it forms a characteristic “V” shape. The apex of this “V” always points in the upstream or uphill direction, opposite to the flow of water. This V-shape rule identifies the direction of water flow and the orientation of valleys. Every contour line must eventually close upon itself, either within the boundaries of the map or somewhere outside the mapped area.