How To Find The Contour Interval Of A Map

How to Find the Contour Interval of a Map: A Comprehensive Guide

Topographic maps are essential tools for hikers, climbers, geologists, and anyone navigating varied terrain. Understanding how to interpret these maps is crucial, and a key element of that understanding is grasping the contour interval. This seemingly simple concept is fundamental to accurately interpreting elevation changes and understanding the three-dimensional landscape represented on a two-dimensional surface. This comprehensive guide will delve deep into the meaning of contour interval, methods for finding it, and practical applications for better map reading.

What is a Contour Interval?

A contour interval is the vertical distance between two consecutive contour lines on a topographic map. Contour lines connect points of equal elevation. Imagine slicing a mountain with equally spaced horizontal planes; each slice represents a contour line. The vertical distance between these slices is the contour interval. Understanding this interval is paramount because it dictates the scale of elevation changes depicted on the map. A small contour interval indicates a detailed representation of subtle elevation changes, while a large contour interval shows a more generalized depiction, focusing on larger-scale changes.

Identifying the Contour Interval: The Easiest Methods

Locating the contour interval is usually straightforward. Mapmakers diligently include this critical information directly on the map itself. Look for:

1. The Map's Legend or Key:

The most reliable place to find the contour interval is the map legend or key. This section usually contains a small box or table summarizing the map's symbols and their meanings. The contour interval will be explicitly stated, often with units (e.g., "Contour Interval: 20 feet," "Contour Interval: 10 meters").

2. The Map's Margin or Title Block:

Sometimes, the contour interval is printed in the map's margin or title block. This area often contains crucial metadata about the map's creation, scale, projection, and other essential information. Scan this area carefully; the contour interval is usually prominently displayed.

3. Inspecting the Contour Lines Themselves (Advanced Method):

If the map lacks clear labeling of the contour interval, you can deduce it through careful observation and calculation. This method requires precision and understanding of map conventions.

• Find two labeled contour lines: Identify two contour lines with clearly indicated elevations.
• Calculate the difference: Subtract the lower elevation from the higher elevation.
• Count the intervening contour lines: Carefully count the number of contour lines between the two labeled lines, including the labeled lines themselves.
• Divide the difference by the count: Divide the elevation difference by the total number of contour lines to find the interval.

Example: Let's say you find two labeled contour lines with elevations of 100 meters and 160 meters. There are five contour lines between them (including the two labeled lines). Therefore, the contour interval is (160-100)/5 = 12 meters. This method relies on accurate line counting and the assumption that the contour lines are consistently spaced, which is usually the case on well-constructed maps.

Understanding Contour Line Characteristics

Beyond simply finding the contour interval, understanding how contour lines themselves behave provides invaluable context for interpreting the map. Here's a closer look:

1. Closely Spaced Contour Lines Indicate Steep Slopes:

When contour lines are clustered closely together, it indicates a steep slope. The closer the lines, the steeper the incline. This is because a small horizontal distance corresponds to a significant elevation change over a short distance.

2. Widely Spaced Contour Lines Indicate Gentle Slopes:

Conversely, widely spaced contour lines represent gentle slopes or relatively flat terrain. A large horizontal distance between lines indicates a smaller elevation change over a longer distance.

3. Concentric Circles Indicate Hills or Mountains:

Contour lines forming concentric circles represent a hill or mountain. The innermost circle indicates the highest point (summit). The spacing of the circles indicates the steepness of the slopes leading to the summit.

4. V-shaped Contour Lines Indicate Valleys or Gullies:

When contour lines form a V-shape, they generally point upstream (against the flow of water). The V-shape indicates a valley or gully, with the apex of the V pointing uphill.

5. Index Contours and Supplementary Contours:

Many topographic maps utilize index contours—heavier or bolder lines—to improve readability. These lines typically have their elevation value printed directly on them, simplifying the interpretation of elevation. Sometimes supplementary contours, thinner lines between index contours, further enhance precision. The interval between index contours is usually a multiple of the basic contour interval (e.g., an index contour every 50 meters with a 10-meter basic interval).

Practical Applications of Knowing the Contour Interval

Understanding the contour interval is more than just a theoretical exercise; it's a crucial skill with numerous practical applications:

1. Determining Elevation Change Between Points:

By knowing the contour interval, you can accurately estimate the elevation difference between any two points on the map. Simply count the number of contour lines between the points and multiply by the contour interval.

2. Planning Hiking or Climbing Routes:

The contour interval directly impacts route planning. It allows hikers and climbers to identify steep sections, assess the difficulty of a route, and plan accordingly. Steep slopes, indicated by closely spaced contours, require careful planning and potentially more time and effort.

3. Assessing Terrain Suitability for Different Activities:

Knowing the contour interval helps determine whether an area is suitable for specific activities. For instance, a gently sloping area (widely spaced contours) might be ideal for camping or cycling, while a steep area (closely spaced contours) might be more suitable for experienced climbers.

4. Understanding Drainage Patterns and Water Flow:

The contour lines reveal information about drainage patterns. Water flows perpendicular to contour lines, always following the path of steepest descent. Understanding this helps predict the course of streams and rivers and potential flooding areas.

5. Geological Interpretations:

Geologists use contour lines to understand landforms, identify geological structures, and interpret geological history. The spacing and patterns of contour lines can provide clues about faulting, folding, erosion, and depositional processes.

Troubleshooting Common Issues

Sometimes finding the contour interval might pose challenges. Here are solutions to typical problems:

1. Missing Map Legend or Title Block:

If the legend or title block is damaged or missing, the advanced method of calculating the interval from labeled contour lines becomes crucial. Remember to meticulously count the lines and check your calculations.

2. Inconsistent Contour Spacing:

While rare, inconsistencies might appear in older or poorly-made maps. If you encounter such an issue, it's essential to carefully assess multiple areas of the map to see if a general pattern or average interval can be established.

3. Unclear Contour Line Labeling:

If the elevation labels on contour lines are faded or illegible, consider using a magnifying glass or other visualization aid. If this doesn't work, you may need to consult additional resources or a more detailed map of the same area.

Conclusion: Mastering the Contour Interval for Enhanced Map Reading

The contour interval is a fundamental element of topographic map interpretation. Knowing how to easily locate it and understanding its implications for interpreting elevation changes and terrain features is a crucial skill for anyone working with topographic maps. By mastering this concept, you significantly enhance your map-reading abilities, leading to safer and more informed decision-making in various outdoor activities and professional applications. This guide provides you with not only the knowledge to find the contour interval but also the understanding to utilize it effectively for improved map reading. Remember, practice makes perfect—the more you work with topographic maps and apply this knowledge, the better you will become at interpreting the landscape they represent.