INTRODUCTION

Contour is an imaginary line joining points of equal elevation.

Contour is mainly used to know the nature of ground in all type of civil engineering work.

Contour is also used to compute the capacity of reservoir.

TERMS USED IN CONTOUR MAP

A) CONTOUR LINE – The line of intersection of level surface with ground surface is called as contour line. These lines join the point of equal reduced level.

CHARECTERISTIC OF CONTOURS

The contour lines are closer near top of hill and and wide apart near the foot (indicating steep slope).

Contours having lower values inside loop indicate ridge line.

A contour having lower value inside the loop indicates a valley line.

Contour lines never cross each other except in case of a overhanging cliff.

A series of closed contour always indicates depression or a summit. The lower value being inside loop indicates a depression.

Uniformly placed contour line indicates a uniform slope.

B)CONTOUR INTERVAL-

The vertical distance between any two consecutive contour is known as contour interval.

The contour interval of map above is 20.

C) HORIZONTAL EQIVALENT- The horizontal distance between any two consecutive contour is known as horizontal equivalent.

METHODS OF CONTOURING

There are two methods of contouring-1. Direct Method –

Case 1 –

1) When area is oblong and cannot be controlled from a single station point then this method is adopted.

2) In this method, the various points on any contour are

located on the ground by taking levels.

3) These points are marked by pegs. After these, the points are plotted on the map, to any suitable scale, by plane table.

4) This method is very slow and tedious but it gives accurate result.

Case 2 –

1) When area is small and can be controlled from a single station point then this method of contouring is adopted.

2) In this method radial lines are drawn in different directions.

3) After that a T.B.M. is established near the site, then the staffman hold the staff along the radial lines until we get required reading.

4) The distance between station point and the position of level staff is measure and then plotted on plane table using suitable scale.

5) This method is very slow and tedious.

2. Indirect Method-A] By using squares-

1) In this method, a base line is considered. Cross-sections are taken perpendicular to this line at regular interval.

2) After that points are marked along cross-section at regular interval of 5m, 10m and so on.

3) All the readings are then entered in level book and the R.L. of each point is calculated.

4) Then base line and cross-sections are plotted to suitable scale after which required contour line is drawn by interpolation.

B] By using squares –

1) In this method the area is divided into number of squares. The sizes of these squares depend upon nature and extent of area.

2) Generally squares have sides varying from 5m to 20m and they are numbered serially.

3) A T.B.M. is setup and the level is setup at suitable position. The staff readings are taken on corners of squares and noted in level book.

4) Then the R.L. of each point is calculated and the skeleton of square is plotted with a suitable scale.

5) After that the contour lines are drawn by the method of interpolation.

REFRANCES-1) Google.co.in

2) Wikipedia.com

3) Surveying and levelling book

(Author- N.N.Basak)