Interpolation of contour-grade-contour-method-of-contour

Contouring

Content

1. Interpolation of contour 
2. By estimation 
3. By arithmetic 
4. By graphical 
5. Grade contour 
6. Procedure for establishing the grade contour on the ground 
7. Methods of contouring 
8. Direct method 
9. Indirect method 
10. Square method 
11. Cross-section method 
12. Tacheometric method 

Interpolation of contour 

The procedure of spacing the contour equally among plotted the ground point is known as interpolation of contour.Interpolation of contour 

The principle of interpolation of contour is based on slope of the ground among two points is homogeneous. There are three methods of interpolation of contour which are as follows:-

1. By estimation
2. By arithmetic 
3. By graphical

Grade contour 

It is the contour establish on a specific grade or gradient along the hill side.Grade contour in surveying 

The line joining the points of equal grades or gradient is termed as grade contour.

Procedure for establishing the grade contour  on the ground:-

The grade contour  along the hill side can be establishing the following procedure:-

1. Suppose a grade contour of 1 in 30 is to be established on the ground. The points of grade contour can be marked approximately using abbey level (Clinometer).
2. By setting the instrument of tripod to all temporary Establishing grade contour adjustments. Take the reading of benchmark of R.L say 100 m as back side reading 0.4 meter, so height of instrument will be 100.4 m.
3. Therefore,  R.L of point (40 m peg intervals in the straight line). Then will be 100.4 - 40/30 =99.067 m. Therefore, to get this R.L on ground, the staff reading should be 100.4 - 99.067 =1.33 m. 
4. Now a staff is held 40 meters away from benchmark (B. M) up & down movement to get 1.33 m readings and then point is marked on the ground with peg. 
5. The above procedure is continue in the same step line, and corresponding points are marked on the ground.
6. Finally, the line joining all the marked point give us the required great contour  of 1 in 30 accurately.

Methods of contouring 

There are mainly two methods of locating the contouring :-

1. Direct method
2. Indirect method

1. Direct method of contouring 

This method is very accurate but slow and complex. In the direct method the contour to the located directly in the field by locating and making a number of points on each contour. In this method a lot of time is wasted in searching the point Indirect Method of contouring of same elevation. 

This is suitable for a small area where greater accuracy is required.

Direct method is suitable for hilly area.

2. Indirect method of contouring 

This method is generally adopted for all kinds of survey and this method is cheaper, quicker, less complex as compared to direct method. 

In this method same guide points are selected along the system of straight line, and their elevations are found. By using the interpolation, contours are drawn by plotting the points which are selected. 

Generally there are three methods :-

1. Square method
2. Cross - section method 
3. Tacheometric method 

1. Square method of contouring 

Square method is used when area to be survey is small and ground is not very much undulating. Square method is also termed as aspat level method.

A number of square is formed on the whole area and vary from 5 m to 25 m and side of square hinge on the nature of ground, nature of contour, contour  interval, scale of the map, etc.

First of all, by the using of levelling instrument and staff we can calculate the height of the corner of a square. After then corner of the square mark on the ground, and then contour of  desired values of drawn by interpolation.

2. Cross - section method of contouring 

This method is suitable for narrow areas just like road, railway, canal, etc.  

In this method cross-section lines are drawn perpendicular to the center line of area. The spacing of cross-section depending upon the nature of ground, nature of contour, contour interval, scale of map, etc.  In general spacing of cross section is taken 20 meters in hilly area and 100 meters in flat area. After this elevation of all points are determined by levelling instrument and staff then contour of desired values are drawn by interpolation.

3. Tacheometric method of contouring 

In this case of hilly area the tacheometric  method is used.

In a tacheometric method a number of radial lines at the position of points at equal distances are marked. After then R.L of all points are calculated and then desired values of contour lines are drawn.

NOTE :-

1. This method is a particularly suitable when contour map of hilly area is required.
2. Number of radial lines are depends upon the nature of ground. 

For more information go through below link 

Classification of surveying 

Accessories of linear measurement

What is field work and field work book 

Principle of surveying 

Contouring

Some questions related from contouring 

1. How many ways to do contouring? 

A. One

B. Two 

C. Three

D. Four 

Correct answer : B. Two

Direct and indirect method 

2. Direct method is suitable for 

A. Hilly area 

B. Plain area

C. Both 

D. None of these 

Correct answer : A. Hilly area

3. The spacing of cross-section in hilly area is usually 

A. 5 m

B. 10 m

C. 15 m

D. 20 m

Correct answer : D. 20 m

4. Contour lines cross valley lines at

A. 30°

B. 45°

C. 60°

D. 90°

Correct answer : D. 90°

5. The points on a contour gradient will have the same elevation 

A. True 

B. False

Correct answer : B. False