You use a contour map to understand and visualize the terrain, primarily by interpreting the contour lines which represent elevation.
A contour map provides a two-dimensional representation of a three-dimensional landscape. Its fundamental purpose is to show the shape and elevation of the land surface.
Understanding Contour Lines
The core element of a contour map is the contour line.
- What they are: Contour lines connect points that share the same elevation above a standard reference point, usually sea level.
- Key Rule: Contour lines never intersect. If they did, it would mean a single point has two different elevations simultaneously, which is impossible.
Reading Terrain from Contour Lines
The spacing and pattern of contour lines tell you a lot about the terrain:
- Slope Steepness:
- Where they're close together, elevation is changing rapidly in a short distance, and the terrain is steep.
- Where contour lines are wide apart, elevation is changing slowly, indicating a gentle slope.
- Hills and Summits: Closed loops of contour lines usually indicate a hill or mountain peak. The highest point is inside the innermost loop.
- Valleys and Depressions: Contour lines that form V-shapes often indicate valleys. The V-points typically point upstream (uphill). Depressions (like craters or sinkholes) can be shown with closed loops that have hachure marks (small lines pointing inward) on the downhill side.
- Ridges: Contour lines that form U-shapes often indicate ridges, with the U-points pointing downhill.
Practical Uses of a Contour Map
Using a contour map allows for various practical applications:
- Navigation: Essential for hikers, climbers, and explorers to plan routes, avoid overly steep areas, and understand where they are in relation to terrain features.
- Planning: Useful for construction, engineering, and land development to identify buildable areas, drainage patterns, and the amount of earth moving required.
- Analyzing Water Flow: Contour lines show the slope, which helps determine the direction water will flow downhill. Valleys (V-shapes pointing uphill) are common drainage paths.
- Estimating Elevation: You can determine the approximate elevation of any point on the map by locating it relative to the nearest contour lines.
- Calculating Elevation Change: By noting the elevation of contour lines, you can calculate the total vertical change over a specific distance along a route.
How to Use a Contour Map Effectively
Here's a simplified guide:
- Identify the Contour Interval: Look for the contour interval on the map's legend or margin. This number tells you the difference in elevation between adjacent contour lines.
- Locate Index Contours: Some contour lines are bolder and labeled with their elevation; these are index contours and make reading elevations easier.
- Trace Contour Lines: Follow a specific contour line to see the path of constant elevation.
- Observe Spacing: Pay close attention to how close or far apart the lines are to understand slope.
- Interpret Patterns: Look for closed loops, V-shapes, and U-shapes to identify hills, valleys, and ridges.
By interpreting the patterns and spacing of contour lines, you can gain a detailed understanding of the terrain without ever setting foot on it.
