# What Is The Contour Interval Of The Map?

A contour interval is a crucial element when it comes to understanding and interpreting topographic maps. It refers to the vertical distance or difference in elevation between contour lines on a map, allowing us to visualize the variations in ground elevation or depression.

By analyzing contour lines, we can gain valuable insights into the shape and features of the land, such as hills, ridges, streams, and depressions. Contour lines play a vital role in conveying this information. Index contours, which are bold or thicker lines, appear at every fifth contour line, making them easily distinguishable.

The numbers associated with contour lines indicate the elevation of the terrain, with increasing numbers indicating an increase in elevation and decreasing numbers indicating a decrease. Additionally, contour lines tend to turn upstream as they approach a stream, canyon, or drainage area.

But how do we determine the contour interval itself? The contour interval of a map can be calculated by subtracting the elevation numbers of two adjacent contour lines. This helps us understand the vertical distance represented by each contour line.

Moreover, contour lines can assist us in determining slope percent and horizontal ground distance, providing a comprehensive view of the terrain. Overall, the contour interval is an indispensable component of a topographic map, enabling us to analyze the elevation and understand the landscape it represents.

### Key Takeaways:

• A contour interval represents the vertical distance between contour lines on a topographic map.
• Index contours, which are bolder lines, occur at every fifth contour line.
• The elevation numbers associated with contour lines indicate the terrain’s elevation.
• Contour lines turning upstream can indicate the presence of a stream, canyon, or drainage area.
• The contour interval can be calculated by subtracting the elevation numbers of two adjacent contour lines.

## How to Read Contour Intervals on a Map?

To effectively navigate and interpret a topographic map, it is essential to understand how to read contour intervals. Contour intervals provide valuable information about the elevation changes and variations in the terrain. By following a few simple steps, you can easily decipher the contour intervals on a map and gain insights into the landscape.

1. Locate the labeled contour lines: Look for two adjacent contour lines on the map that are labeled with specific elevations. These lines will serve as reference points for calculating the contour interval.
2. Calculate the difference in elevation: Determine the vertical distance between the two contour lines. This difference in elevation will help you determine the contour interval.
3. Count the non-index contour lines: Identify the number of contour lines that are not bold or thicker (non-index lines) between the two index contour lines. Add 1 to this count.
4. Divide the difference in elevation: Divide the difference in elevation between the two index contour lines by the sum obtained in the previous step. This will give you the contour interval for that specific topographic map.

Once you have determined the contour interval, you can use it to interpret the shape and features of the land. Contour lines that are close together indicate steep slopes, while lines that are further apart represent gentle slopes. Understanding contour intervals allows you to identify hills, ridges, depressions, and other topographical features on the map.

By following the contour lines, you can estimate the elevation of intermediate points and obtain a better understanding of the terrain’s elevation changes. The patterns and spacing of the contour lines provide crucial information about the landscape’s characteristics.

With practice, reading contour intervals becomes second nature, enabling you to navigate and interpret topographic maps with confidence.

Key Points to Remember
Contour intervals indicate elevation changes on a topographic map.
The difference in elevation between two contour lines determines the contour interval.
Count the non-index contour lines, add 1, and divide the difference in elevation to calculate the contour interval.
Interpreting contour intervals helps identify the shape and features of the land.
Following contour lines allows estimation of intermediate point elevations.

## Factors Affecting the Selection of Contour Interval

When creating a topographic map, several factors need to be considered to determine the appropriate contour interval. These factors play a significant role in ensuring the accuracy and usability of the map for various purposes.

The selection of the contour interval depends on the scale of the map, extent of survey, nature of the ground, and the time and resources available. The scale of the map is an essential factor to consider when deciding the contour interval. Smaller scale maps that cover a larger area with varying terrain require a higher contour interval.

This is because a higher contour interval will adequately represent the elevation changes over a larger area without overcrowding the map with too many contour lines. On the other hand, larger scale maps that focus on a smaller area can use a lower contour interval to provide more detailed information about the terrain.

The extent of the survey also influences the selection of the contour interval. For rough topographical maps used for initial assessments, a higher contour interval may be sufficient. These maps are often created quickly and serve as a general overview of the terrain.

In contrast, detailed maps used for execution work require a smaller contour interval to capture the finer details and variations in the topography. The nature of the ground is another factor that affects the contour interval selection.

If the ground has significant variations in levels such as hills, valleys, or ponds, a smaller contour interval may be necessary to accurately represent these features on the map. Conversely, if the ground is relatively flat with minimal elevation changes, a higher contour interval can be used without compromising the map’s readability.

Lastly, the availability of time and resources can impact the decision-making process. Creating maps with a smaller contour interval requires more time and resources to accurately survey and collect data. If time and resources are limited, a higher contour interval may be chosen as a more practical option.

### Table: Factors Affecting Contour Interval

FactorsDescription
Scale of the mapDetermines the size of the area covered and influences the choice between a higher or lower contour interval.
Extent of surveyAffects the level of detail required in the map, with rough assessments using higher contour intervals and execution work using smaller intervals.
Nature of groundThe presence of significant variations in levels may require a smaller contour interval to accurately represent the terrain’s features.
Time and resources availableLimited time and resources may result in the selection of a higher contour interval for practical reasons.

## Uses of Contour Intervals in Surveying

Contour intervals play a crucial role in surveying, especially when mapping large areas. They help in efficiently representing the elevation changes on a small piece of paper. When dealing with larger areas, a higher contour interval is utilized, while smaller areas require a smaller contour interval to showcase more intricate details.

In order to maintain readability in larger maps, the use of index contour lines is preferred. Another significant application of contour intervals is determining the elevation of intermediate points between labeled contour lines. This is especially valuable in surveying projects where precise elevation measurements are essential.

Contour intervals are also key in estimating earthwork quantities for various structures, such as bridges, dams, and roads. By analyzing the contour lines, surveyors can accurately calculate the amount of earth that needs to be moved or excavated, enabling efficient project planning and execution.

Additionally, contour intervals aid in calculating horizontal distances on a map, also known as the horizontal equivalent. This allows surveyors to determine the actual dimensions of a project area, considering both the vertical elevation changes and the horizontal distances between different points.

It’s important to note that contour intervals are based on vertical levels, while horizontal equivalents focus on horizontal distances and require careful scaling and measurement on the map.

## FAQ

### What is a contour interval?

A contour interval is the vertical distance or difference in elevation between contour lines on a topographic map. It indicates the variation in ground elevation or depression.

### What are index contours?

Index contours, which are bold or thicker lines, appear at every fifth contour line. They help in reading the elevation values and provide a reference point on the map.

### How do I determine the contour interval of a specific topographic map?

To determine the contour interval, locate two adjacent contour lines labeled with specific elevations. Calculate the difference in elevation between these lines and count the number of non-index contour lines between them. Add 1 to this count and divide the difference in elevation by the sum obtained to get the contour interval.

### What can contour lines on a map help me determine?

Contour lines help in interpreting the shape and features of the land, such as hills, ridges, streams, and depressions. They can also assist in estimating slope percent and horizontal ground distances.

### What factors influence the selection of the contour interval for a topographic map?

Several factors influence the selection of the contour interval, including the scale of the map, extent of the survey, nature of the ground, and the availability of time and resources.

### How are contour intervals useful in surveying?

Contour intervals are useful in surveying as they help in representing elevation changes efficiently, estimating earthwork quantities, finding the elevation of intermediate points, and calculating horizontal distances on a map.

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