What is Contour Interval | Factor Affecting it | Consideration | Calculation | Uses -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

## May 26, 2021

CONTOUR INTERVAL

A contour interval is a vertical distance or 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 a particular contour lines are increasing, elevation increases in the area as well.

What is Contour Interval?

Counter interval is the difference between the two consecutive contour lines on the map

Ex: Suppose a contour map having contour lines of 100m, 96m, 92m then

Contour interval= 4m

Factor Affecting Contour Interval

The Contour Spacing Depends On The Following Factors:

1. The Scale of The Map

The contour interval is inversely proportional to the scale of the map. If the scale is small, the contour interval should be large and vice versa.

2. Nature of The Ground

The general terrain of the area determines the contour interval. For a flat area, the contour interval should be small.

For sloping or undulating terrain, the chosen contour interval should be large.

3. Purpose and Expansion of the survey work

If surveying work is to be used for precise and detailed calculations, then the small contour interval should be chosen and a large contour interval is chosen for the following cases:

·       For catchment areas

·       To the reservoir

·       For location survey

4. Time available and Eligible Expenses for field and office work

A large contour interval should be used if the time available for survey work is less. The small contour interval, survey work and plotting need to be more precise. For a small contour interval, the money needed will also be more as the field and office work will be larger.

Consideration When Choosing The Appropriate Contour Interval

Before preparing a contour map, the required contour interval is decided. Taking into account the following factors on which the contour interval depends

·       Surface: The Nature Of The Floor Surface

·       Map Scale

·       Purpose And Extent Of The Survey

·       Time And Expense Are Required

 THE FOLLOWING CONTOUR INTERVALS MAY BE ADOPTED FOR VARIOUS PURPOSE. Small scale maps or rough country 5m to 25 m Large scale maps pr flat country 0.5 m Location survey 2 to 3 m Building sites 0.3 to 0.5 m Reservoir and town planning work 0.5 to 1 m Detailed design of works 0.5 m

Methods of Contouring

There are two methods of locating contour

1.   Direct Method

2.   Indirect Method

Direct method of contouring

This method is difficult and time-consuming. In this mode, the contour is placed directly on the ground, and the points of a given edge are displayed on the ground. Then these points are calculated and found within a certain range of positions on the horizontal plane. You can now plan your map on the map. Fieldwork involved in this method includes:

1.   Take backsight of the BM.

2.   Determine the height of the instrument

3.   Calculate the staff reading required for a particular contour by the relation.

Desired staff reading = HI – contour elevation

4.   Instruct the staffman to move within the local boundaries so that the desired staff read the dumpy level.

5.   Mark the dots on the ground with nails or colored tags.

6.   Use the same position or move the device and repeat the procedure for another contour.

7.   Perform a survey to determine the relative positions of these points on the contour line in the field.

8.   Mark the survey on paper in an appropriate scale.

Indirect Method

In this method, levelling work is carried out on-site, and the location of the contour points is carried out in the office. Site surveys include:

It establishes a clear pattern of ground points with known relative positions.

This method Determine the altitude of these points on the site. The indirect method is cheaper, faster and not boring than the direct method.

Various Indirect Methods are

1.   Method of Cross-section

2.   Method of Squares

3.   Technometrics Contouring

Contour Intervals Calculation

Usually, the map legend identifies the contour spacing on the map, but sometimes only a portion of the map is available. Knowing how to calculate contour interval becomes a useful technique. On most maps, each fifth contour, marked with a thicker or darker line, is an index line or an index contour. This index line is marked highly.

Find the elevation of the two adjacent index lines. The highest number represents an uphill ascent. Find the difference between the two elevations.

For Example,

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

Let’s say, 6000 and 6100 and calculate the interval between them.

Now the difference between 6100 and 6000 is 6100 – 6000 = 100.

The variety of contour lines is 4 between 6000 and 6100.

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.

Uses Of Contour Maps

1.   You can choose a suitable site for your construction activity.

2.   You can determine the reservoir capacity.

3. Alignment of roads, canals and transmission lines is performed efficiently through contour lines.

4.   The characteristics of the surface can be determined.

5.   Earthwork volume estimation.

6.   Ground characteristics (e.g. flatness, uniformity, sloping, undulations or hills, etc.)

7. Using the contour map, you can select the most suitable and economical site for your work, such as roads, railroads, canals, pipelines, dams, and reservoirs.

8.   The amount of reservoir storage water, cuts and embankments can be estimated using contour lines.

9.   You can use contour lines to plan military operations.

10. They are used to obtain data used in the design and construction of irrigation structures such as dams.

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