What is Contour Interval?
What is a Contour Interval?
A contour interval is the vertical distance or the difference in elevation between two contour lines in a topographical map. It is typically determined based on the size of the area being mapped.
Calculating the Contour Interval
The contour interval of a map can be calculated by noting the right-hand bottom side of the map, which should specify the interval. If it is not specified, it can be calculated based on the size of the area to be mapped and the scale of the map. A common contour interval for a 1:24,000 map scale is 20 feet.
Factors Affecting the Selection of Contour Interval
The selection of the contour interval is decided by the survey leader before the start of the mapping process depending upon the ground factors.
| Sl. No | Factors | Select High CI like 1m, 2m, 5m or more | Select Low CI like 0.5m, 0.25m, 0.1m or less |
| 1 | Scale of the map | For small scale maps covering a wide area of varying terrain | For large maps showing details of a small area |
| 2 | Extent of survey | For rough topographical map meant for initial assessment only | If a detailed map is to be prepared for execution work |
| 3 | Nature of ground | If the ground has large variation in levels, for instance, hills and ponds | If the terrain is comparatively level |
| 4 | Time and resources available | If less time and resources are available | If time and resources abundant |
How to Calculate Contour Interval from Maps?
What is a Contour Map?
A contour map is a geographical representation of an area, displaying contour lines that indicate different elevations. Contour maps are used to show elevation, shape, and slope of a given area, allowing viewers to visualize the topography of a region.
What are Index Contour Lines?
Index contour lines are the lines on a contour map that are marked with elevation readings. These marked lines make it easier to read the map and identify different elevations. They are used to provide a reference for the other contour lines to help map readers identify the relative elevations of the area.
Calculating Contour Intervals
Contour lines, also known as index contours, are lines on a map that connect points of equal elevation. These lines are used to represent a three-dimensional land surface on a two-dimensional map. To accurately represent the shape of the land surface, the contour lines must be spaced evenly. Calculating the contour intervals is an important step in the process of creating a map.
Understanding Contour Intervals
The contour interval is the difference in elevation between two adjacent contour lines. This interval is determined by the range of elevation across the map. For example, if the highest point is at 1,000 feet and the lowest point is at 500 feet, the range of elevation is 500 feet. To evenly space the contour lines, the contour interval is calculated by dividing the range of elevation by the desired number of contours. In this example, if we want 10 contours, the contour interval is 500 รท 10 = 50 feet.
Creating Contour Lines
Once the contour interval is determined, the contour lines can be created by starting with the highest elevation and counting down by the contour interval. For example, if the highest elevation is 1,000 feet, the contours would be drawn at 1,000, 950, 900, 850, and so on. This process is repeated until all the contour lines are drawn.
Conclusion
Calculating the contour interval is an important step in creating a map. By understanding the range of elevation and the desired number of contours, you can calculate the contour interval and draw the contour lines accurately. This helps to represent the three-dimensional land surface on a two-dimensional map.
Step 1:
Finding Contour Lines
To locate two index contour lines that are labeled with a specific elevation, start by studying a topographic map. Index contour lines are the boldest contour lines on a topographic map with the elevation value written on the line. Locate two index contour lines that are labeled with the same elevation value. The lines should be close together or even crossed. Take note of the location of these lines to determine the elevation of the area.
Step 2:
Calculating the Difference Between Contour Lines
To calculate the difference between two contour lines selected from a map, subtract the higher elevated line from the lower elevated line. This will give you the difference between the two contour lines. The result of this subtraction will be the difference in elevation between the two contour lines.
Step 3:
Step 1: Calculating the Contour Interval
In order to calculate the contour interval, two index contour lines must be selected. The difference between the elevations of the two index contour lines is the contour interval.
Step 2: Counting the Non-Index Contour Lines
Once the contour interval has been determined, the number of non-index contour lines between the two index contour lines can be counted. These lines are usually labeled with their respective elevations and can be used to measure the exact elevation of points along the contour line.
Step 4:
COUNTING LINES
To count the number of lines between two index lines, first locate the two index lines. Then count the number of lines between the two index lines. Finally, add 1 to the number of lines obtained to get the final result. For example, if there are 5 lines between two index lines, the final result would be 6.
Step 5:
Step 1: Calculate the Difference between Two Index Lines
To calculate the difference between two index lines, subtract the lower index line from the higher index line.
Step 2: Divide the Difference by the Number of Lines
Divide the difference between two index lines (calculated in Step 1) by the number of lines in between the two index lines.
Step 3: Determine the Number of Lines between Two Index Lines
Count the number of lines between the two index lines. This should include the two index lines, as well as any lines in between.
Step 4: Divide the Difference by the Number of Lines Plus One
Divide the difference between two index lines (calculated in Step 1) by the number of lines in between the two index lines, plus one.
Step 5: Find the Quotient
The final step is to find the quotient of the difference between two index lines (calculated in Step 2) and the number of lines in between two index lines plus one (calculated in Step 4).
Step 6:
- Calculating The Contour Interval
The contour interval of a topographical map is calculated by dividing the difference between the highest and lowest elevations on the map by the number of contour lines. By dividing the difference in elevation between the highest and lowest points on the map by the number of contour lines, we can determine the contour interval of the map.
- Understanding the Contour Interval
The contour interval on a topographical map is an important factor to consider when reading the map. It indicates the vertical distance between the lines on the map, which can be used to determine the elevation of different areas on the map. The contour interval helps to understand the topography of an area and provides useful information for hikers and other outdoor activities.
- Final Answer
The final answer we get after dividing is the contour interval of the specific topographical map. This value will give us the vertical distance between each contour line on the map, which will help us better understand the topography of the area.
Example Calculation of Contour Intervals:
Steps Involved in Contour Interval Calculation
- Calculate the interval: The interval between two contour lines can be calculated by taking the difference between the two elevations. For example, the interval between 7000 and 7100 is 7100 – 7000 = 100.
- Calculate the number of contour lines: To calculate the number of contour lines in-between the two elevations, count the number of contour lines in the map. In this example, there are 4 contour lines in-between 7000 and 7100.
- Calculate the Contour Interval: To calculate the contour interval, add 1 to the number of contour lines and divide the difference between the two elevations by the result. In this example, 4+1=5 and 100/5=20 units. Hence, the Contour Interval of the above map is 20 Units.
Uses of Contour Intervals in Surveying
What is Contour Interval Mapping?
A contour interval is a technique used in mapping to represent the shape and elevation of an area. It uses contour lines to indicate changes in the terrain, and the corresponding contour intervals show the amount of elevation change between each line. Contour intervals are especially useful when mapping large areas, as they allow for a more detailed visualization of changes in elevation.
What are the Benefits of Contour Interval Mapping?
Contour interval mapping offers several advantages, including the ability to accurately measure vertical changes in elevation, and to calculate earthwork estimates for structures like bridges, dams and roads. Additionally, contour intervals are often used to identify areas of steep slopes, and to determine the most efficient paths for transportation, such as road and railway planning.
What is the Difference Between Contour Intervals and Horizontal Equivalents?
Contour intervals measure the vertical elevation between two points on two consecutive contour lines, while horizontal equivalents measure the horizontal distance between two points on the same contour line with a given slope. This is especially useful when mapping large areas, as the use of index contour lines allows for the calculation of intermediate points elevation.
| S. No | Contour Interval | Horizontal Equivalent |
| 1 | It is based on vertical levels | Represents horizontal distance |
| 2 | No measurement or scaling is required since the contour levels are indicated on the contour lines | The distance must be measured on the map and converted to actual distance by multiplying with the scale of the map |
| 3 | In a given map the contour interval is a constant | The horizontal equivalent varies with slope. Closer distance indicates steep slope and wider distance gentle slope |