What Is A Contour Interval

Understanding Contour Intervals: A Comprehensive Guide

Contour lines are the unsung heroes of topographic maps, silently conveying crucial information about the three-dimensional shape of the land. Understanding contour intervals is essential for interpreting these maps effectively, whether you're a hiker planning a route, a surveyor assessing land features, or a geologist studying terrain formations. This comprehensive guide will demystify contour intervals, explaining what they are, how they're used, and why they're so important. We'll delve into the practical applications, explore common misconceptions, and answer frequently asked questions, providing a solid foundation for anyone working with topographic maps.

What is a Contour Interval?

A contour interval is the vertical distance between consecutive contour lines on a topographic map. It represents the difference in elevation between one contour line and the next. For example, a contour interval of 20 feet means that each contour line is 20 feet higher in elevation than the one below it. This consistent interval is crucial because it allows for accurate determination of elevation and slope across the mapped area. The contour interval used on a map is usually indicated in the map's legend or key.

Imagine slicing a landscape with a series of perfectly horizontal planes, each spaced at a fixed vertical distance. The intersections of these planes with the land surface are then projected onto the map, creating the contour lines. The spacing between these lines directly reflects the contour interval. Closely spaced contour lines indicate a steep slope, while widely spaced lines suggest a gentle slope or flat terrain.

How Contour Intervals are Determined

Choosing the appropriate contour interval is crucial for map readability and accuracy. Several factors influence this decision:

• Scale of the map: Larger scale maps (showing smaller areas in greater detail) generally use smaller contour intervals to represent subtle elevation changes. Smaller scale maps covering larger areas often use larger intervals to avoid overcrowding the map with numerous contour lines.

• Terrain relief: Areas with significant elevation changes require larger contour intervals to maintain map clarity. Conversely, relatively flat areas may utilize smaller intervals to depict subtle variations in elevation.

• Purpose of the map: Maps intended for specific purposes (e.g., hiking, engineering, geological studies) may necessitate different contour intervals depending on the level of detail required. For instance, a map for hiking might use a smaller contour interval to show subtle changes in terrain important for trail planning.

• Data availability: The precision of the elevation data used to create the map affects the smallest practical contour interval. More precise data allows for smaller intervals.

Interpreting Contour Lines and Intervals: A Practical Guide

Contour lines themselves provide a wealth of information, and understanding their characteristics in conjunction with the contour interval is paramount:

• Elevation: The elevation of each contour line is determined by adding or subtracting the contour interval from the elevation of a known contour line. Often, a benchmark point (a point of known elevation) is indicated on the map to provide a starting point for this calculation.

• Slope: The spacing of contour lines directly indicates the steepness of the slope. Closely spaced lines mean a steep slope, while widely spaced lines indicate a gentle slope. Very closely spaced lines might even suggest a cliff or a near-vertical drop.

• Relief: The total vertical distance between the highest and lowest points in the mapped area is called the relief. The contour interval and the number of contour lines present on the map together determine the relief.

• Depressions (closed contour lines with hachures): Contour lines enclosing a lower elevation than surrounding lines indicate a depression or a sinkhole. These are usually marked with short, inward-pointing lines called hachures.

• Ridges and valleys: Contour lines form a "V" shape when they cross a stream or valley, with the "V" pointing uphill. On ridges, the "V" points downhill.

• Hills and summits: Contour lines form closed loops around hills and summits. The highest point is indicated either by a label or by the highest elevation contour line.

Beyond the Basics: Advanced Concepts and Applications

While the core concept of contour intervals is relatively straightforward, its applications extend into more complex areas:

• Calculating gradients: The contour interval and the distance between contour lines on the map (measured using the map scale) can be used to calculate the gradient (slope) of the land. A steeper gradient is indicated by closer contour lines for a given contour interval.

• Creating three-dimensional models: Contour lines serve as the basis for creating three-dimensional models of the land surface, using various computer software and techniques.

• Hydrological analysis: Contour lines are essential for understanding drainage patterns, identifying watersheds, and assessing potential flood risks.

• Engineering applications: Contour lines are critical for road design, construction projects, and other engineering tasks that require an accurate understanding of the terrain.

• Geological mapping: Geologists use contour lines to map geological formations, fault lines, and other geological features. The contour interval helps them to understand the three-dimensional structure of rock formations and the relationship between different rock units.

Common Misconceptions about Contour Intervals

• Contour lines never cross: This is a fundamental rule. If contour lines cross, it implies that a particular point on the landscape has two different elevations simultaneously, which is physically impossible.

• Contour lines always close upon themselves: While this is often true, particularly for summits and depressions, contour lines may run off the edge of the map if the feature they represent extends beyond the map boundaries.

• Contour interval remains constant across the entire map: While the contour interval is intended to be constant, variations might occur due to inaccuracies in the original elevation data or errors in map production. However, significant deviations are rare and should be considered carefully.

Frequently Asked Questions (FAQ)

Q: What happens if the contour interval is too large or too small?

A: A contour interval that is too large will obscure subtle terrain features, leading to loss of detail and potentially misinterpretations of slope and relief. Conversely, a contour interval that is too small can lead to map overcrowding, making the map difficult to read and interpret.

Q: How can I determine the elevation of a point between two contour lines?

A: This requires interpolation. You can estimate the elevation by visually judging the point's position between the two lines and proportionally allocating the contour interval. More sophisticated techniques may be used for greater accuracy.

Q: Can I use contour maps to plan a hiking route?

A: Absolutely! Contour lines are invaluable for planning hiking routes, as they reveal the steepness of the terrain. Careful study of the contour lines allows for choosing the easiest and safest routes, avoiding overly steep ascents and descents.

Q: Are contour intervals the same on all topographic maps?

A: No. Contour intervals vary depending on the scale of the map, the terrain relief, and the intended purpose of the map. The contour interval is always clearly stated on the map's legend.

Q: What is the difference between a contour line and a spot height?

A: A contour line connects points of equal elevation. A spot height is a point on the map with its elevation explicitly marked, often indicated by a small black dot and the elevation value. Spot heights are helpful in refining the interpretation of contour lines.

Conclusion

Understanding contour intervals is fundamental to interpreting topographic maps effectively. This knowledge allows for accurate determination of elevation, assessment of slopes, identification of landforms, and planning routes, all critical for diverse applications ranging from hiking and surveying to geological studies and engineering projects. By mastering the concepts explained in this guide, you'll unlock a new level of understanding and appreciation for the valuable information contained within these often-overlooked cartographic tools. Remember to always check the map's legend to identify the contour interval before embarking on any interpretation. With practice and attention to detail, the secrets of the landscape revealed by contour lines will become clear and readily accessible.