A contour interval is a vertical distance or a difference in elevation between contour lines in a topographic map, index contour lines are bold or thick lines that appear on every fifth contour line and if the number associated with specific contour lines is increasing, the elevation of the area is also increasing.

For the plotting of region, the contour intervals are assumed, the contour intervals is specified on the right-hand side.

When the contour interval is not specified within the map, it may be calculated within the following sections, the generally used contour interval is 20 feet for a 1: 24,000 map scale.

## Factors Affecting the Selection of Contour Interval:

The choice of the contour interval is determined by the survey leader before the beginning of the mapping process based on ground factors as follows.

Factors: | Select High CI like 1m, 2m, 5m or more. | Select Low CI like 0.5m, 0.25m, 0.1m or less. |

Scale of the map: | For small scale maps covering a large area of different areas. | For large maps showing details of a small area. |

Extent of the survey: | For rough topographic map means for preliminary assessment only. | If a detailed map is to be prepared for the execution work. |

Nature of ground: | If there’s a massive change in ground levels, for example, hills and ponds. | If the terrain is relatively level. |

Time and resources available: | If much less time and resources can be found. | If time and resources are considerable. |

### How to Calculate Contour Interval from Maps?

A contour map consists of contour lines of a given geographic area.

To maintain the contour map easy and simple to learn, every contour line is not marked by its elevation studying, these marked or labelled lines are identified or known as index contour lines.

The calculation of the contour intervals is as follows:

**Step 1:**

First, find the two index contour lines which can be labelled with a particular height.

**Step 2:**

Calculate the difference between the 2 selected index contour lines from a map.

To take the difference, subtract the upper elevation line with lower elevation line.

**Step 3:**

Now calculate the number of non-index lines contour lines between the two indexed contour lines chosen for the contour interval calculated in the 1st step.

**Step 4:**

The variety of lines obtained within the above step is taken and added with 1.

For example: if the variety of rows between 2 index lines is 5, then add 1 to 5 which turns into 6.

**Step 5:**

The ultimate step is the quotient of the difference between the index lines (step 2) and the variety of lines between the 2 index lines plus 1 (step 5).

**Step 6:**

The final answer is obtained by dividing the contour intervals of the particular topographic map.

#### Example contour interval calculation:

Considering the map, the following steps are used to calculate the contour intervals calculation as follows:

Let’s say, 7000 and 7100 and calculate the interval between them.

Now the difference between 7100 and 7000 is 7100 – 7000 = 100.

The variety of contour lines is 4 between 7000 and 7100.

Add 1 to 4, 4 + 1 = 5

Now divide 100 by 5,

100/5 = 20 units

The contour interval of the above map gave us 20 units.

#### Difference between Contour interval & Horizontal equivalent:

Contour interval | Horizontal equivalent |

The contour interval is used to calculate the vertical elevation of a location. | The horizontal distance between two factors on two contour lines for a given slope is known as the horizontal equivalent. |

It represents the vertical distance. | It represents the horizontal distance. |

Measurement or scaling is not required because contour levels are indicated on contour lines. | The distance needs to be measured on the map and multiplied with the dimensions of the map to be transformed to actual distance. |

The contour interval in a given map is constant. | The horizontal level varies with the slope, nearer distances indicate a steep slope and a wider distance signifies a mild slope. |

### Uses of Contour Intervals in Surveying:

- Contour interval is used when a big space is to be mapped to a small piece of paper.
- A higher contour interval is used for a bigger area and shorter contour intervals for a smaller area.
- In a bigger map, index contour lines are much less to maintain the map easy to learn.
- To find the height of the intermediate point, the contour interval is used.
- Earthquake estimation for bridge, dam, or road can be found with the help of contour intervals within the map.

Also read: Chain Surveying, Tacheometry, Plane Table Surveying & Total Station

### Conclusion:

In short, contour interval is a vertical distance or difference in height between contour lines whereas index controls are bold or thick lines that appear on each fifth contour line.