# What is Contour Interval?Contour Interval Applications in Surveying

## What is a contour interval?

A contour interval is the distance between two adjacent contours on a map. This distance is typically measured in feet or meters, but can also be measured in other units of measure. The contour interval is an important factor in determining the resolution of a map.

A contour interval is the distance in which measurements exist for a particular point on the surface of that consists of two parts: the horizontal and vertical distances between consecutive points.

In surveying, a contour interval is the vertical distance or elevation difference between two contour lines on a topographical map. Typically, various contour intervals are used for different maps.

Contour intervals are assumed based on the size of the area to be mapped. The contour interval is specified on the right-hand bottom side of each map.

It is possible to determine the contour interval when it is not provided on the map. For a 1:24,000 map scale, the standard contour interval is 20 feet.

## Factors Affecting the Selection of Contour Interval

Contour interval is affected by the contour interval of the map scale, the area being mapped, the topographic features being mapped and time and resources available.

### The Contour Interval of a Map Scale

The contour interval of a map scale represents an idealized distance between two points on a surface. To obtain an actual value, first calculate the length of each degree on the map, then multiply that length by 360 to determine the total length measured along one degree’s worth of latitude or longitude.

The same process is used to determine the total number of degrees measured on each degree’s worth of latitude and longitude.

The resulting interval will represent an idealized distance based on map scale, but will be very close in reality due to the approximate nature of the technique.

### The Area to Be Mapped

The size of the area being mapped has an effect on how closely a contour interval should approximate actual elevation measurement, but is not an absolute requirement. In areas with dense cloud cover, the contour interval will be lower than in areas with high above-ground features.

In areas with dense vegetation, it may be difficult to distinguish between two different profiles of a single hill.

### The Topographic Features to Be Mapped

In mountainous terrain, the most interesting regions will be higher and where valleys and ridges protrude far below the surface.

In flat or urban areas, the contour interval will be smaller because features are more spread out and easier to distinguish.

### Time and resources available

If time is the limiting factor, then it may be best to focus on areas with the highest possible levels of detail.

## Calculation of Contour Intervals

The contour interval on the map is normally identified by a map legend, but sometimes only a portion of a map is visible.

Understanding how to compute the contour interval is a useful skill to have. Each fifth contour line, indicated as a heavier or darker line on most maps, is an index line or index contour. The elevation of these index lines will be marked.

Determine the heights of two adjacent index lines. The highest number represents the steepest ascent. Determine the difference in elevations between the two.

For example, if the uphill height is 1,000 feet above sea level and the lowest elevation is 800 feet above sea level, the elevation difference is 200 feet.

To Understanding how to compute the contour interval is a useful skill to have. Begin by counting the contour lines from one index line to the next. Maps typically count five contour lines, including the next index line, from one index line to the next.

The contour intervals are calculated as follows:

Step 1: First, find two index contour lines that are identified with an elevation.

Step 2: Now compute the difference between the two index contour lines chosen from a map. Subtract the upper elevated line reading from the lower elevated line reading to get the difference.

Step 3: Count the number of non-index contour lines that exist between the two index contour lines that were chosen for the contour interval calculation in the first step.

Step 4: Take the number of lines acquired in the previous step and multiply it by 1. For example, suppose there are 5 lines between two index lines. Then multiply 1 by 5 to get 6.

Step 5: the last step is the quotient of the difference between two index lines (step 2) and the number of lines between the two index lines plus one (step 5).

Step 6: The contour interval of the specified topographical map is the last response we get after dividing.

## How to find Contour Interval

Example:

The contour interval calculation, assume two index contour lines, 7000 and 7100, and determine the interval between them.

7100 – 7000 = 100 is the difference between the two selected index contour lines 7100 and 7000.

Assuming the number of non-index lines contour lines between 7000 and 7100 is four.

The number of lines obtained in the preceding step is 4 added by 1.

4 + 1 = 5 when added together

Now divide 100 by 5, and you’ll get 20 units.

## Contour Interval Applications in Surveying

The contour interval is used in surveying for the following reasons:

• When mapping a huge space to a little piece of paper, Contour Interval is used.
• The horizontal equivalent is the horizontal distance between two spots on two consecutive contour lines with a given slope.
• As contour intervals are used to calculate an area’s vertical elevation, a comparable method for calculating horizontal distance is known as Horizontal Equivalent.
• The contour intervals on the map can be used to estimate earthquakes for a dam, bridge, or road.
• The contour interval is used to determine the height of the intermediate point.
• Index contour lines are reduced in size on larger maps to keep the map simple to memorize.
• A larger contour interval is used for a larger area, while a smaller contour interval is used for a smaller area.

## The difference between Contour intervals and Horizontal Equivalent

Because contour intervals are used to compute the vertical elevation of a region, the same method is used to calculate the horizontal distance, which is known as Horizontal Equivalent.

Horizontal equivalent is the horizontal distance between two spots on two consecutive contour lines with a given slope.

The following shows the difference between Contour intervals and Horizontal Equivalent:

1. Contour Interval is based on vertical levels, whereas Horizontal Equivalent represents horizontal distance.
2. Contour Interval requires no measurement or scaling because the contour levels are indicated on the contour lines, whereas Horizontal Equivalent requires measuring on the map and converting to real distance by multiplying by the map’s scale.
3. The contour interval in a particular map is fixed, whereas the horizontal equivalent varies with slope. The closer the distance, the steeper the slope, and the larger the distance, the gentler the slope.

## How to Use Smaller Contour Interval

A smaller contour interval is used for a smaller area, while a larger contour interval is used for a larger area.

The larger the contour interval, the more precise the survey. If a smaller contour interval is used, increase the survey scale.

If a larger contour interval is used, increase the survey scale only if necessary to maintain adequate accuracy of depiction.

The smaller the value of the contour interval, the easier it is too parallel read on your map and make accurate measurements. Therefore, use a larger contour interval to give you precise reading and measurement.

## How to Use a Larger Contour Interval

A larger contour interval is useful for the following situations:

1. If a detail survey is needed.
2. If two or more maps will be used together, the largest contour interval should be used for all maps.
3. It is easier to read and provides more detail than a smaller contour interval on your map.
4. If you are measuring a long distance on your map, a larger contour interval will let you see the smaller details on the map.
5. If it is easier to read and understand the detail on the map than a smaller contour interval, and if additional detail is needed for accurate depictions.
6. The larger contour interval will be suitable for maps with shaded relief or patterns, or when using text symbiology with low angles of elevation such as 5° and 10°.
7. If the map has a lot of detail in the area, such as streets or contour lines.

## Advantage of using a smaller contour interval

A smaller contour interval is suitable for surveys with a large area and it is easier to read and understand the detail on the map than a larger contour interval.

If you have a small amount of detail to measure like road lines, lakes and streams, a smaller contour interval will give you more detail.

## Disadvantages of using a smaller contour interval

If you are looking for fewer steep slopes on your map, a smaller contour interval will make the map look rougher and you’ll have to estimate the slope.

## How to Use Faster Contour Interval

If you want to use less time when surveying, use a larger contour interval. A larger contour interval is useful for the following situations:

1. If your map has a lot of detail in the area, such as streets or contour lines.
2. If you have a map with shaded relief or a pattern, such as rivers and canals.
3. If you are drawing trees and contour lines, use a larger contour interval.
4. If you are drawing rivers and canals and rendering heights of intermediate points, use a larger contour interval.

## FAQs

### What is a contour interval?

A contour interval is the horizontal distance between two consecutive contour lines. It is used in surveying to determine the vertical elevation of an area with a given slope.

### Why do we have contour lines on maps?

Contour lines are used in surveying to map out the terrain. They are essential for the accurate mapping of land contours to determine elevation and geologic structure. The contour interval is used to calculate the vertical elevation of a region and thus, it’s very important for topographic cartography.

### Why do some people use smaller contour intervals?

A smaller contour interval gives more detailed information on your map, which is useful when you’re making measurements with a small distance. It’s also easier to read and understand the detail on your map.

### Why do we use a larger contour interval?

A larger contour interval lets you see more detail on your map, which is useful when you’re making measurements with a large distance. It’s also easier to read and understand the detail on your map.

### Why do I use contour intervals when calculating elevation?

You use contour intervals for two reasons:

• If you are measuring a long distance on your map, a larger contour interval will let you see the smaller details on the map.
• If it is easier to read and understand the detail on the map than a smaller contour interval, and if additional detail is needed for accurate depictions.

### Do I have to use a larger contour interval?

No. It is not necessary to use a larger contour interval. However, a larger contour interval will make your map look more natural.

### How should I decide what Contour Interval is best for me?

Always remember that bigger intervals mean more detail and smaller intervals mean less detail.

Smaller Contour Interval: most accurate, but harder to read and understand the detail on the map.

### What is the contour interval shown on the map?

Individual contour lines on a topographical map are separated by a set elevation interval known as a contour interval.

Contour intervals of 5, 10, 20, 40, 80, or 100 feet are common. The contour interval of a map is determined by the terrain being portrayed as well as the map’s scale.

### How do you find the contour interval?

The contour interval is shown on your map as the horizontal distance between two successive contour lines.

For example, a 5-foot contour interval would indicate 5′ intervals between contour lines. A 10-foot contour interval would indicate 10′ intervals between contour lines, and so on.

### How do you read the elevation?

When reading and measuring elevations, use a larger interval whenever possible.

### What is contour interval apex?

A contour interval is the distance between two consecutive contour lines. Interval is expressed in meters or feet (and inches). The contour interval apex is the elevation of the highest point of a “V” shaped line between two contour lines.

### What is a contour index?

A contour index is the ratio of the contour interval to a given length. The contour index of a map is determined by its scale and by the extent of the map’s depiction.

### How do you read contour intervals?

To read a contour interval, count how many intervals there are between two consecutive lines. In metric units, use two fingers for each interval; in English units, use one finger for each interval. For example, a contour interval of 5 miles would be one finger; a contour interval of 4 miles would be two fingers; and so on.

### What are the intervals between lines of a contour map?

The intervals between lines are the horizontal distance between two consecutive lines. The contour intervals are usually in meters or feet between two lines. Read the height in light rain, heavy rain and snow also depending upon area to be mapped.

### What is the highest elevation on a topographical map?

The highest point, or the summit, of a “V” shaped line between two contour lines is known as the contour interval apex.

For example, if you are on a contour interval of 100 feet and you go up to the top of a “V”, then you are at the contour interval apex.

### What is a contour interval?

A contour interval is the vertical distance or elevation difference between contour lines. Index contours are thicker or more prominent lines that emerge every fifth contour line. If the numbers linked with specific contour lines rise, the height of the landscape rises as well.