## Lesson 13. Contours- Map preparation

The art of determining relative altitudes of points on the surface of the earth of beneath the surface of earth is called LEVELLING.

A contour is defined as an imaginary line of constant elevation on the ground surface. It can also be defined as the line of intersection of a level surface with the ground surface. For example, the line of intersection of the water surface of a still lake or pond with the surrounding ground represents a contour line.

Indirect method of contouring:

In this method, the spot levels of selected guide points are taken with a level and their levels are computed. The horizontal positions of these points are measured or computed and the points are plotted on the plan. The contours are then drawn by a process called interpolation of contours from the levels of the guide points. The following are the indirect methods are commonly used for locating contours.

1. Squares or Grid method

2. Cross section method

Square or grid method:

In this method, the area to be surveyed is divided into a grid or series of squares. The grid size may vary from 5 m x 5 m to 25 m x 25 m depending upon the nature of the terrain, the contour interval required and the scale of the map desired. Also, the grids may not be of the same size throughout but may vary depending upon the requirement and field conditions. The grid corners are marked on the ground and spot levels of these comers are determined by leveling. The grid is plotted to the scale of the map and the spot levels of the grid corners are entered. The contours of desired values are then located by interpolation. Special care should be taken to give the spot levels to the salient features of the ground such as hilltops, deepest points of the depressions, and their measurements from respective corners of the grids, for correct depiction of the features. The method is used for large scale mapping and at average precision.

Cross section method:

In these sections, a base line, centre line or profile line is considered.  Cross sections are taken perpendicular to this line at regular intervals.  After this points are marked along the cross sections at regular intervals.  A temporary bench mark is set up near the site.  Staff readings are taken along the base line and the cross sections. The readings are entered in the level book the base line and the cross sections should also be mentioned.  The RL of each of the points calculated.  Then the base line and cross sections are plotted to a suitable scale.  Subsequently the RLs of the respective points are noted on the map, after which the required contour line is drawn by interpolation

This method is suitable for route survey, when cross sections are taken transverse to the longitudinal section.

Method of interpolation of contours:

The process of locating the contours proportionately between the plotted points is termed interpolation.  Interpolation may be done by:

1. Arithmetical calculation

2. The graphical method

By arithmetical calculation

Let A and B be two corners of the squares.  The RL of A is 98.75 m, and that of B 100.75 m.  the horizontal distance between A and B is 10m.

Horizontal distance between A and B = 10m

Vertical difference A and B = 100.75-98.75=2m

Let a contour of 99.00 m be required.  Then,

Difference of level between A and 99.00m contour = 99.00-98.75=0.25m

Therefore, distance of 99.00 m contour line form A= 10/2 *0.25=1.25m

This calculated distance is plotted to the same scale in which the skeleton was plotted to obtain a point of RL of 99.00 m.

Similarly, the other points can be located.

By graphical method

On a sheet of tracing paper, a line AB is drawn and divided into equal parts.  AB is bisected at C and a perpendicular is drawn at this point.  A point O is selected on this perpendicular line and then radial lines are drawn from O to the divisions on AB.  After this lines 1-1, 2-2, 3-3….are drawn parallel to AB.  These lines serve as guide lines.  Boundary line and every fifth the line is marked with a thick or red line.

Suppose we have to interpolate a 2m contour between two points a and b of RLs 92.5 and 100.75m.

Let us consider the lowest radial line OB to represent an RL of 90.00. So, every fifth line will represent 95,100,105, etc.  The tracing paper is moved over the plan until ‘a’ lies at 92.5 and ‘b’ at 100.25. Line ‘ab’ should be parallel to AB.  Now the points 94, 96, 98,100 are picked through to obtain the positions of the required contours.

Method of interpolation of contours:

The process of locating the contours proportionately between the plotted points is termed interpolation.  Interpolation may be done by:

1. Arithmetical calculation

2. The graphical method

By arithmetical calculation

Let A and B be two corners of the squares.  The RL of A is 98.75 m, and that of B 100.75 m.  the horizontal distance between A and B is 10m.

Horizontal distance between A and B = 10m

Vertical difference A and B = 100.75-98.75=2m

Let a contour of 99.00 m be required.  Then,

Difference of level between A and 99.00m contour = 99.00-98.75=0.25m

Therefore, distance of 99.00 m contour line form A= 10/2 *0.25=1.25m

This calculated distance is plotted to the same scale in which the skeleton was plotted to obtain a point of RL of 99.00 m.

Similarly, the other points can be located.

By graphical method

On a sheet of tracing paper, a line AB is drawn and divided into equal parts.  AB is bisected at C and a perpendicular is drawn at this point.  A point O is selected on this perpendicular line and then radial lines are drawn from O to the divisions on AB.  After this lines 1-1, 2-2, 3-3….are drawn parallel to AB.  These lines serve as guide lines.  Boundary line and every fifth the line is marked with a thick or red line.

Suppose we have to interpolate a 2m contour between two points a and b of RLs 92.5 and 100.75m.

Let us consider the lowest radial line OB to represent an RL of 90.00. So, every fifth line will represent 95,100,105, etc.  The tracing paper is moved over the plan until ‘a’ lies at 92.5 and ‘b’ at 100.25. Line ‘ab’ should be parallel to AB.  Now the points 94, 96, 98,100 are picked through to obtain the positions of the required contours.