In this article, we will discuss
What is a contour interval | What are Contour Lines | How Can We Use of Contour Map | Factors Affect Contour Interval | how to find contour interval
What is a contour interval?
A contour interval is a vertical distance between two contour lines on a topographical map or the difference in elevation between them.Most of the time, there are different contour intervals for each map. Contour intervals are based on the area that needs to be mapped. On the right side of the bottom of every map, the contour interval is written. Most maps with a scale of 1:24,000 use a contour interval of 20 feet.
To calculate the contour interval, divide the elevation difference between the index contour lines by the number of contour lines between the index contour lines.
For instance, if the distance of 200 is divided by the number of lines, where the number of lines is 5, the result is 5. The contour interval is equal to 200 / 5 = 40, or contour intervals of 40 units.
What are Contour Lines
Contour lines are lines on a map that connect points of the same height. Contour lines show the height of the land and how it is shaped.The surface of the mountain is shown by the order of the contour lines going up, and the cover of the valley or ridge is shown by the order of the contour lines going down.
Index Contour
On a topographical map, one of the ways that vertical height is shown is through the use of something called an index contour.For the purpose of making it easier to identify, one of the contour lines depicted on the map is kept at a greater vertical difference than the other contour lines.
Contour Gradient
In a mountainous area, contour gradient is the name for the lines that connect points with the same slope.How Can We Use of Contour Map
- Contours are used to help make maps.
- Knowing the topography of the land helps when using contour.
- Contour can be used to make military strategies.
- Contour is used to choose where to build dams, bridges, and other things.
- The contour map can help you figure out how much water is in the reservoir.
- From the contour map, the contour gradient is made.
- The contour map can be used to figure out how big the closed discharge field is.
- With contour maps, you can find out how big a reservoir is and which part is at the bottom.
- Used to make a cross on the contour map in any direction.
- Very helpful for figuring out how long a road, pipeline, sewer line, railroad track, etc.
- The contour map helps you figure out how high two points are.
Characteristics of Contour Lines
- All of the points on the contour line are at the same height.
- Two contour lines that are at different heights don't meet. But the lines for the overhanging cliffs cross each other.
- But if the slope of the land is steep, the contour lines are far apart.
- And if the slope of the land is low, the contour lines are close to each other.
- In a plain, the lines that make up its shape are straight and parallel to each other.
- Even though the lines of the raised land are crooked and uneven,
- If the plains all have the same slope. If so, the distances between the contour lines are the same.
- As the height goes from the outside to the inside, the lines close up to show a hill mark.
- It's a pit mark if the height goes down from the outside to the inside in closed contour lines.
- The saddle is the bottom part of the hill. Four pairs of contour lines represent it.
- Contour lines with the same height don't cross each other.
- The map line that shows the shape of the land ends where it began.
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Factors Affect Contour Interval
Nature of Ground
- If the ground slopes steeply, contour lines are far apart.
- If the ground slope is low, contour lines are close. Plains have parallel contour lines.
The scale of the Map
Contour spacing increases as the map scale decreases. Contour spacing decreases as the map scale increases.Purpose of Surveying
1. Topographical Survey
Determining natural features of rivers, lakes, forests, and artificial features of roads, railways, cities, villages, etc.
In this method, the level machine is put in the middle, then it is levelled, and then the benchmark is read first. Then, they read on the ground. Let's say the standard is R.L. 100 mt and the reading is 1.40 mt.
Now we have to draw a 100-mt ground contour line. So that each point on the 100 mt contour line has a staff reading of 1.4 mt. So, marking the different points on the ground with a 1.40 mt reader and connecting them makes a 100 mt R.L. contour line.
On the map, contour lines can be drawn. In this way, the level machine keeping staff at different points can figure out the relative heights.
The indirect method has three ways to do things.
2. Cadastral Survey
- property line determination.
- Calculate area.
- state and municipality boundaries.
3. City Survey
- city road alignment.
- Water and sewer line alignment.
- River discharge.
- Beach and river survey.
- Coast flight measurement.
Method of Contouring
There are two ways to shape a surface,
- Direct method
- Indirect method
1. Direct Method
A dumpy-level machine is used to draw the lines between the points that are all the same height.In this method, the level machine is put in the middle, then it is levelled, and then the benchmark is read first. Then, they read on the ground. Let's say the standard is R.L. 100 mt and the reading is 1.40 mt.
Now we have to draw a 100-mt ground contour line. So that each point on the 100 mt contour line has a staff reading of 1.4 mt. So, marking the different points on the ground with a 1.40 mt reader and connecting them makes a 100 mt R.L. contour line.
2. The indirect way
The direct method takes longer and costs more than this method. With less work, an indirect method can be used.On the map, contour lines can be drawn. In this way, the level machine keeping staff at different points can figure out the relative heights.
The indirect method has three ways to do things.
- Method Of Squares.
- Method Of Cross-Sectioning
- Method Of Tacheometry.
how to find contour interval
Step 1.
First, we have to find two index contour lines that are at a certain height.Step 2.
Now, figure out how far apart the two chosen index contour lines are. To see the difference, subtract the reading from the higher line from the reading from the lower line.
Step 3.
We need to figure out how many non-index contour lines are between the two index contour lines we chose in step 1 for the contour interval we found.
Step 4.
The number of lines found in the step above is written down and 1 is added to it. Let's take a simple example: If there are five lines between two index lines, The result will be 6 if you add 1 to 5.
Step 5.
The last and final step is to divide the distance between two index lines by the number of lines between the two index lines (step 5).
Step 6.
After dividing, we get the contour interval of the topographical map we were given.
Rules of Contour Intervals
- Contour lines show which way is up and which way is down.
- Unless they meet at a cliff, contour lines never touch or cross each other.
- The colour of every fifth contour line is darker than the other colours. This line shows the shape of an INDEX. It's called the index contour line.
Benefits of Contour Interval
- It shows how steep things are and how big they are on the map.
- When you look at the contour intervals, it's easy to figure out the different heights of the landscape.
- It can be used to make cross-sections of certain parts of a mapped area.
Drawbacks of Contour Interval
- Due to the size of the vertical interval, the contour can't show all of the heights.
- Some of the shapes of the land can't be shown using contours. Like a coral reef, a rock outcrop, or a crater.
- The contour method isn't used to show relief on a small-scale map because it might make some details hard to see.