What Is The Contour Interval On This Map

Let's unravel the mystery of contour intervals and learn how to decipher the language of topographic maps. Understanding contour intervals is essential for anyone wanting to grasp the three-dimensional shape of the Earth's surface represented on a two-dimensional map. This article provides a comprehensive guide to understanding contour intervals on maps, covering what they are, how to interpret them, and why they are so important.

What is a Contour Interval?

A contour interval is the vertical distance or difference in elevation between adjacent contour lines on a topographic map. Contour lines are imaginary lines that connect points of equal elevation above a reference datum, usually mean sea level. The contour interval remains constant for a specific map and is usually indicated in the map legend. It's expressed in units of measurement such as feet or meters, depending on the map's scale and geographic location.

For example, if a map has a contour interval of 20 feet, each contour line represents an elevation that is 20 feet higher or lower than the adjacent contour line. Therefore, if one contour line is labeled 100 feet, the next contour line could represent either 80 feet or 120 feet.

Importance of Contour Intervals

Contour intervals are essential for understanding the topography of an area displayed on a map. They provide critical information about the steepness and shape of the terrain. Here are several reasons why contour intervals are important:

Terrain Analysis: Contour intervals allow map readers to visualize the landscape's three-dimensional form. Closely spaced contour lines indicate steep slopes, while widely spaced lines indicate gentle slopes. This information is invaluable for planning activities like hiking, construction, or military operations.
Elevation Determination: By examining contour lines and their intervals, one can determine the elevation of any point on the map. If a point lies between two contour lines, its elevation can be estimated using interpolation.
Hydrological Studies: Contour lines help identify watersheds and drainage patterns. Water flows perpendicular to contour lines, moving from higher to lower elevations. The contour interval assists in understanding how water moves across the landscape, aiding in flood risk assessments and water resource management.
Construction and Engineering: In civil engineering, contour maps are used for site planning, road design, and infrastructure development. Understanding the contour interval is crucial for calculating cut and fill volumes, determining slope stability, and ensuring proper drainage.
Navigation and Orientation: Hikers, climbers, and other outdoor enthusiasts rely on contour intervals to navigate unfamiliar terrain. By understanding the slope and elevation changes, they can plan routes that are both safe and efficient.

Reading and Interpreting Contour Lines

To accurately interpret a topographic map, it is crucial to know how to read and interpret contour lines. Here are some key principles to remember:

Contour Lines Never Cross: Except in rare cases such as overhanging cliffs or caves, contour lines never intersect. If they appear to merge, it usually indicates a very steep or vertical slope.
Contour Lines Form Closed Loops: Contour lines eventually close upon themselves, either on or off the map.
Concentric Closed Contours Indicate Hills or Depressions: Closed contours typically represent hills or depressions. If the elevation increases toward the center, it's a hill. If the elevation decreases toward the center, it's a depression. Depressions are sometimes marked with hachure lines (short lines pointing inward).
Uniform Spacing Indicates Uniform Slope: If contour lines are evenly spaced, it means the slope is consistent.
Irregular Spacing Indicates Varied Slope: Varying spaces between contour lines show changes in slope. Closely spaced lines indicate steep areas, while widely spaced lines show gentle slopes.
Contour Lines Bend Upstream: When contour lines cross a stream or river, they form a "V" shape pointing upstream (uphill). This is because the land slopes down towards the stream.
Index Contours: To make maps easier to read, some contour lines are thicker and labeled with their elevation. These are known as index contours and typically occur at regular intervals (e.g., every fifth contour line).

Determining the Contour Interval on a Map

The contour interval is typically specified in the map legend or marginal information. However, if it's not explicitly stated, you can determine it by following these steps:

Locate Two Adjacent Index Contours: Find two index contours (the thicker, labeled lines) on the map.
Note Their Elevation Values: Record the elevation value of each index contour.
Count the Number of Contour Lines Between Them: Count the number of contour lines (excluding the index contours themselves) that lie between the two index contours.
Calculate the Elevation Difference: Subtract the lower elevation from the higher elevation to find the total elevation difference between the index contours.
Divide by the Number of Intervals: Divide the total elevation difference by the number of intervals (which is one more than the number of contour lines between them). This will give you the contour interval.

Example:

Let's say you find two index contours on a map. One is labeled 500 feet, and the other is labeled 600 feet. You count four contour lines between them.

Elevation difference = 600 feet - 500 feet = 100 feet
Number of intervals = 4 contour lines + 1 = 5
Contour interval = 100 feet / 5 = 20 feet

Therefore, the contour interval on this map is 20 feet.

Factors Affecting the Choice of Contour Interval

The choice of contour interval depends on several factors, including the map scale, the terrain's relief, and the map's purpose. Here are some considerations:

Map Scale: Large-scale maps (e.g., 1:24,000) typically have smaller contour intervals because they show more detail over a smaller area. Small-scale maps (e.g., 1:250,000) usually have larger contour intervals to avoid overcrowding the map with too many lines.
Terrain Relief: Areas with low relief (flat or gently sloping) may use smaller contour intervals to accurately represent subtle changes in elevation. Areas with high relief (steep mountains) usually use larger contour intervals to avoid contour lines becoming too dense.
Map Purpose: The intended use of the map also influences the choice of contour interval. For example, a map used for detailed engineering design may require a smaller contour interval than a map used for general navigation.

Common contour intervals include 5, 10, 20, 40, 80, and 100 feet in the United States. In metric countries, common intervals are 1, 2, 5, 10, 20, and 50 meters.

Examples of Contour Interval in Different Terrains

To better understand how contour intervals are used in various terrains, let's look at some examples:

Flat Terrain: In flat areas, such as coastal plains or river valleys, a small contour interval (e.g., 5 feet or 1 meter) is used to show slight variations in elevation. Contour lines will be widely spaced, indicating gentle slopes.
Hilly Terrain: In rolling hills, a moderate contour interval (e.g., 20 feet or 5 meters) is appropriate. Contour lines will be more closely spaced than in flat terrain, showing the steeper slopes of the hills.
Mountainous Terrain: In steep mountainous areas, a large contour interval (e.g., 100 feet or 30 meters) is used. Contour lines will be very closely spaced, indicating the steep slopes of the mountains. Cliffs may be represented by contour lines that are very close together or even merge.
Depressions: Depressions, such as craters or sinkholes, are indicated by closed contour lines with hachure marks pointing inward. The elevation decreases toward the center of the depression.

Practical Applications of Contour Intervals

Understanding contour intervals has numerous practical applications in various fields. Here are some notable examples:

Hiking and Backpacking: Hikers use contour maps to plan routes, estimate the difficulty of trails, and identify potential hazards. By analyzing the contour interval, they can determine the steepness of slopes and choose routes that match their fitness level.
Civil Engineering: Engineers use contour maps to design roads, bridges, and other infrastructure projects. The contour interval helps them calculate cut and fill volumes, ensure proper drainage, and assess slope stability.
Urban Planning: Urban planners use contour maps to assess the suitability of land for development. Understanding the topography helps them plan residential areas, parks, and other amenities while minimizing environmental impact.
Agriculture: Farmers use contour maps to manage irrigation, prevent soil erosion, and optimize crop yields. Contour plowing, for example, involves plowing along contour lines to create furrows that trap water and reduce runoff.
Environmental Management: Environmental scientists use contour maps to study watersheds, assess flood risks, and monitor the impact of climate change on landscapes.

Common Mistakes to Avoid

When working with contour intervals, it's essential to avoid common mistakes that can lead to misinterpretations. Here are some pitfalls to watch out for:

Assuming Constant Slope: Just because contour lines are evenly spaced in one area doesn't mean the slope is constant throughout the entire map. Always examine the spacing of contour lines in different parts of the map.
Ignoring the Map Legend: The map legend contains crucial information about the contour interval, scale, and other map features. Always consult the legend before interpreting the map.
Failing to Account for Index Contours: Index contours are essential for quickly determining elevations on the map. Make sure to use them as reference points when analyzing the topography.
Confusing Hills and Depressions: Remember that closed contour lines can represent either hills or depressions. Look for hachure marks to identify depressions.
Neglecting the Units of Measurement: Be mindful of the units of measurement used for the contour interval (e.g., feet or meters). Using the wrong units can lead to significant errors in elevation calculations.

Advanced Techniques for Working with Contour Intervals

For those who want to delve deeper into the analysis of topographic maps, here are some advanced techniques:

Interpolation: Interpolation is the process of estimating the elevation of a point that lies between two contour lines. Linear interpolation assumes a constant slope between the contour lines and can be used to approximate the elevation.
Contour Profiling: Contour profiling involves creating a cross-sectional view of the terrain along a specific line. This can be done by plotting the elevation of each contour line along the profile line and connecting the points.
Slope Calculation: The slope of the terrain can be calculated using the contour interval and the horizontal distance between contour lines. The slope is typically expressed as a percentage or a ratio.
3D Modeling: Digital elevation models (DEMs) can be created from contour maps using specialized software. These models allow for three-dimensional visualization and analysis of the terrain.

Contour Interval and Digital Elevation Models (DEMs)

In modern mapping and GIS (Geographic Information Systems), contour intervals are closely related to Digital Elevation Models (DEMs). A DEM is a digital representation of ground surface topography. DEMs can be generated from various sources, including:

Contour Maps: Contour lines from topographic maps can be digitized and converted into a DEM.
LiDAR (Light Detection and Ranging): LiDAR is a remote sensing technology that uses laser light to measure the elevation of the Earth's surface with high precision.
Photogrammetry: Photogrammetry involves creating three-dimensional models from aerial photographs.
Radar Interferometry: Radar interferometry uses radar signals to measure changes in elevation over time.

DEMs are used for a wide range of applications, including terrain analysis, flood modeling, landslide hazard assessment, and visualization. They provide a continuous representation of the terrain, unlike contour maps which only show specific elevation levels.

The accuracy and resolution of a DEM depend on the source data and the method used to generate it. DEMs derived from high-resolution LiDAR data will generally be more accurate and detailed than those derived from contour maps with large contour intervals.

The Future of Contour Intervals and Topographic Mapping

While digital mapping technologies have advanced significantly in recent years, contour intervals remain a fundamental concept in topographic mapping. They provide a simple and effective way to represent the three-dimensional shape of the Earth's surface on a two-dimensional map.

In the future, we can expect to see further integration of contour intervals with digital mapping tools and technologies. For example, augmented reality (AR) applications could overlay contour lines on real-world views, allowing users to visualize the topography in real-time.

Furthermore, machine learning algorithms could be used to automatically extract contour lines from satellite imagery and other remote sensing data, making it easier to create and update topographic maps.

Conclusion

Understanding the contour interval is essential for anyone who wants to interpret topographic maps effectively. By learning how to read and interpret contour lines, you can gain valuable insights into the shape, steepness, and elevation of the terrain. Whether you are a hiker, engineer, urban planner, or environmental scientist, a solid grasp of contour intervals will enhance your ability to analyze and understand the world around you. So, take the time to study contour maps, practice your skills, and unlock the secrets of the landscape.