What Is a Contour Interval?
Contour Interval Definition: A contour interval is a vertical height or a difference in elevation between contour lines in a topographic map. Usually, there are different contour intervals for the different vertical distances on maps.
Define Contour interval is a line that connects the different points of equal height at the surface of the earth.
Example: The level of the calm water surface is the same. If its R.l is 200 mt, then 200 mt shows the contour.
A contour line is a line on a map representing an imaginary line on the earth’s surface, with all points at the same height as the datum plane, usually considered the sea surface as a datum plane.
Contour lines are contours drawn on a map connecting points of equal elevation. Every Contour lines show elevation and the shape of the terrain.
The increasing order of contour lines represents the mountain’s surface, and the decreasing of contour intervals represents the cover of the valley or ridge.
An index contour is one of the ways that vertical height is demonstrated on a topographical map.
One contour line shown on the map is kept at a higher vertical difference than other contour lines for ease of identification.
What Is a Contour Gradient?
All lines connecting points of uniform slope in a mountainous area are called contour gradient.
Use of Contour Map
- Maps are drawn with the help of contours.
- With the help of contour, knowing the topography of the ground helps.
- Military tactics can be created using Contour.
- Contour is used to select the location for the construction of dams, bridges, etc.
- The reservoir’s capacity helps to know from the contour map.
- The contour gradient is drawn from the contour map.
- The area of the closed discharge field is be determined from the contour map.
- Contour maps can determine the area of a reservoir, which area lies in the lower part.
- Used to draw a cross in any direction from the contour map
- Very useful for determining line length of the road, pipeline, sewer line, railway line, etc.
- The contour map helps to know the height of a given two points.
Characteristics of Contour
- The vertical distance of all the points on the contour line is equal.
- Two contour lines of different heights do not intersect each other. But the contour line for overhanging cliff intersects each other.
- But if the sloping of the ground is high, then the contour lines are far apart from each other.
- And if the slope of the ground is low, then the contour lines are very close to each other.
- The contour lines of a plain are straight and parallel to each other.
- While the contour lines of the elevated ground are crooked and irregular
- If the slope of the plains is uniform. If so, the contour lines are at equal distances.
- As the height increases from the outside to the inside, it shows a hill mark in closed contour lines.
- If the height decreases from the outside to the inside in closed contour lines, it indicates a pit mark.
- The lower part of the hill is called the saddle. Four pairs of contour lines represent it.
- Contour lines of similar height do not intersect.
- The contour line on the map ends at the point where it started.
Contour Interval Depends on the Following Factors.
1. Nature of Ground:
But if the sloping of the ground is high, then the contour lines are far apart from each other.
And if the slope of the ground is low, then the contour lines are very close to each other. The contour lines of a plain are straight and parallel to each other.
2. Scale of the Map:
The smaller the scale of the map, the higher the contour spacing. The larger the scale of the map, the smaller the contour spacing.
3. Purpose of Surveying
3.1. Topographical Survey: Determining natural features rivers, lakes, forests, etc., and artificial features road, railway, cities, villages, etc.
3.2. Cadastral Survey:
- To determine the property line.
- To calculate the area.
- To determine the boundary of state and municipality.
3.3. City Survey:
- To determine the alignment of city roads.
- To determine the alignment of water lines and sewer lines.
3.4. Marine Survey:
- To know the discharge of the river.
- Beach and river survey.
- To measure the flight of the coast.
3.5. Military Survey:
Surveys are conducted on military matters.
4. Area of Surveying
The smaller the area of surveying, the smaller the contour interval distance.
The larger the area of surveying, the high the contour interval distance.
The Contour Intervals for Different Purposes Are Kept as Follows
|1||Large Scale||Building Site||0.2 to 0.5|
|2||Medium Scale||Town planning||0.5 to 5.0|
|3||Small Scale||Cadastral Survey||5 to 10|
Method of Contouring
The method of contouring occurs in two ways,
- Direct method
- Indirect method
1. Direct Method
In this method, the points connecting the points of equal height are drawn using a dumpy level machine.
In this method, the level machine is fixed in the center, then the machine’s leveling is done than reading is done on the benchmark first. Then reading is done on the ground. Suppose the benchmark is R.L 100 mt, and It comes with a reading of 1.40 mt.
We now have to draw a ground contour line of 100 mt. So that the staff reading of each point on the 100 mt contour line is 1.4 mt. Thus marking the different points with a 1.40 mt reader on the ground and connecting them becomes a contour line with 100 mt R.L.
2. Indirect Method
This method is faster and cheaper than the direct method. With less effort from an INDIRECT METHOD,
contour lines can be drawn on the map. In this way, relative heights are determined by level machine keeping staff at different points.
There are three ways of the Indirect method
- Method Of Squares.
- Method Of Cross-Sectioning
- Method Of Tacheometry.