# What is Contouring in Surveying – Contour Lines & Contour Intervals

## What is the definition of contouring in surveying?

In surveying, contouring is the process of determining the elevation of numerous places on the ground and putting these points in the same horizontal positions on a contour map.

Vertical control leveling work is being performed while horizontal control chain survey, compass survey, or plane table survey is being performed.

When using a theodolite, both horizontal and vertical controls can be achieved with the same instrument. Contouring can be classified into numerous groups based on the equipment utilized.

## Contouring Survey Overview

Contour surveying is the branch of surveying that uses contour lines to identify variations in the height of the earth’s surface and their relative places. The goal of contour surveying is to determine the altitude or height of any surface.

A contour survey, in its most basic form, depicts the elevation changes over your property at regular intervals, from the lowest point to the highest one.

Contour lines connect places that are at the same height. The steeper the part, the closer the contour lines are together.

These contour lines are critical to people who want to buy land or a piece of property and are often referred to as the “survey” for the property.

In other words, the contour survey shows the elevation of a piece of land or property as outlined by curved lines – this is why it is also known as ‘the form plan’.

Even though contour lines help determine if a piece of land is level or not, this does not mean that topography has nothing to do with it.

If your property is at the same level as a neighboring piece of land, it will have the same topography. If it is not, it will have its own. If it sits on a hill, it will have its own elevation.

It is through this process of experiencing and understanding changes in elevation that you start to see how topography plays an important role in determining the shape of your piece of land. That is why contour lines are so important.

## What is a Contour line in surveying?

A contour line is an imaginary line that passes between places of equal dedication. A contour line is also described as the intersection of a level surface and the earth’s surface.

Submarine Contours, Fathoms, or Bathymetric Curves are the names given to contours drawn underwater.

This is the ideal approach for representing features such as hills, depressions, undersea, and so on a two-dimensional piece of paper.

### Characteristics of Contour Lines in Surveying.

The Characteristics of Contour Lines are:

1. The topographical map is a true representation of the contour lines.
2. The contour lines are at a fixed interval, which is easy to work with.
3. At higher elevations, you get steeper lines on the map and these may not be the correct ones.
4. The contour lines are going up vertically from point to point which means that rounded corners do not show up in these maps
5. The topography of the land is not shown accurately with contour lines. This can be prevented by using a topographic map with contour lines given at intervals of 10 feet rather than 20 feet.
6. Contours are affected by aerial photographs and maps, which can cause the lines to appear as if they are distorted, even though they are not.
7. The length of the contour interval is from point to point and this means that radii do not show up on these maps.

### Types of Contour Lines in Surveying and their Importance.

The different types of Contour Surveying and their importance are:

• Isopleth- On a contour map, an isopleth connects locations that have the same value of a measurable quantity of geographical or meteorological events.
• Isohyet- Isohyet denotes the points of equal rainfall in a particular region over a specific time period.
• Isobar- Isobars on contour maps represent the point at which atmospheric pressure is equal or constant for a specific period.
• This sort of contour is used to forecast future weather patterns. Isobars are frequently used in weather reporting on television.
• Isobath- It is a form of imaginary contour line on a map or chart that links all places with the same depth under the water’s surface, such as the ocean, sea, and lake.
• Isohaline- It is a form of contour line on a map that connects points of equal salinity in a body of water (majorly contour).
• Isotherm- It is a form of contour line on a map or chart that links points of equal temperature in a certain area. They are further classified into two categories.
• Isocheim: This is a line with the same mean winter temperature.
• Isothere: This is a line of equal summer temperature.

The freezing level is an isotherm at 0 degrees Celsius

• Isohel- It is a form of contour line on a map that links locations with similar amounts of sunlight for a certain period.
• Isohume -It is a form of contour line on a map that links all of the points in a specified area with equal relative humidity.
• Isoneph- It is a straight line that links all places with an equal level of cloud cover.
• Isopectic-Isopectic lines are imaginary lines drawn on a contour map or chart that link all of the sites where ice begins to develop at the same moment.

### Different Types of Contour Lines

On a map, there are three types of contour lines: intermediate, index, and supplemental.

• Index lines- are the thickest contour lines and are typically designated with a number at a single location along the line.

This informs you how high you are above sea level.

• Intermediate- The thinner, more common lines between the index lines are known as intermediate lines.

They normally don’t have a label with a number on it. In most cases, one index line appears for every five intermediate lines.

• Supplementary lines-Are the ones which indicate flatter terrain, show as dotted lines.

### Purpose of Contour Lines

Contour lines are used to show the three-dimensional shape of the terrestrial surface on a two-dimensional map.

Contour lines are formed by the intersection of a horizontal plane parallel to the reference level and the to-be-described topographical surface. As a result, contour lines are always closed curves.

Contour lines are useful because they allow us to map the shape of the land surface (topography).

The diagrams below depict the same island. The diagram on the left depicts a side view (cross profile view) as seen from a ship offshore.

### Contour Lines: How They Work

Contour lines are the feature that allows this to happen: The steepness of the ground is shown by contour lines.

Contour lines connect locations that have the same elevation: where they are near together (they never intersect), elevation changes quickly over a short space and the landscape is steep.

### 5 Contour Lines Rules

They are:

Rule 1: Every point on a contour line has the same elevation.

Rule 2: Contour lines separate uphill and downhill slopes.

Rule 3: Except near a cliff, contour lines should not touch or cross each other.

Rule 4: Every fifth contour line should be darker in color. This is a contour line with an INDEX.

Rule 5 – In steep terrain, contour lines are closer together, while in flat terrain, they are farther away.

## What is a contour interval?

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 the contour interval on a map

Contour intervals indicate the difference in height between two contour lines. The contour interval can be found in the map key, which is normally positioned beneath the scale of the map at the bottom center.

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 Intervals and Their Applications in Surveying

Contour intervals are used to map a huge area onto a small piece of paper. For a wide area, a larger contour interval is utilized, while for a small area, a smaller contour interval is used.

Contour intervals on a map can be used to calculate earthwork estimates for any sort of structure, such as bridges, dams, or roadways.

Because the contour intervals are used to calculate the vertical elevation of a region in the same way that they are used to calculate the horizontal distance, they are referred to as the horizontal equivalent.

### How to Read Topographic Map Contour Intervals

A contour line is a line drawn on a topographic map to show the elevation or depression of the terrain. A contour interval is the vertical distance or elevation difference between contour lines.

1. Index lines are the thickest contour lines, and they are typically designated with a number at one point along the line. This informs you how high you are above sea level.
2. The intermediate lines are the thinner, more common lines that connect the index lines. They normally don’t have a label with a number on it. In most cases, one index line appears for every five intermediate lines.
3. Supplementary lines are shown as dotted lines to indicate smoother terrain. When looking at an index line, the elevation is easy to see because it is plainly stated. Interval lines, on the other hand, are a little more difficult.

You’ll need to know the contour intervals to figure out their elevation.

## 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.

### 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.

### What is contour line?

A contour line is an imaginary line that passes between places of equal dedication. A contour line is also described as the intersection of a level surface and the earth’s surface.

### What are contour lines used for?

These are fictitious lines that connect points of equal elevation, allowing you to read the form of the Earth’s surface. You can measure the steepness of a hill, the height of a mountain, and even the depth of a lake or ocean by reading contour lines.

### What is contour line in Levelling?

These are fictitious lines that connect points of equal elevation, allowing you to read the form of the Earth’s surface. Tell students that they can measure the steepness of a hill, the height of a mountain, and even the depth of a lake or ocean by reading contour lines.

### How do contour lines determine elevation?

Levelling is the art of determining the relative heights of places on the earth’s surface or beneath the earth’s surface. A contour is a fictitious line of constant elevation on the ground surface.

It can alternatively be described as the point at where a level surface intersects the ground surface.

### Can contour lines overlap?

They may get quite close to one other (for example, along a cliff), but they must never cross each other. This is due to the fact that one point on the Earth’s surface cannot be at two different heights.

### How do you tell if contour lines are going up or down?

Remember that contour numbering reads uphill – that is, the number at the top is uphill and the number at the bottom is downhill. Also, keep in mind that the closer the contour lines are together, the steeper the slope.

### What is the map’s 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.