By Graphical Method

This technique is rapid and convenient while high accuracy is needed and interpolation work is too much. In that method, a tracing paper/cloth is used to draw parallel lines at some fixed intervals, say 0.5 m, at equal intervals and every 10th line is made thicker. Now let A and B are two points of elevation 57.5 m and 68.5 m, respectively. Assume it is needed to interpolate 5 m contours between A and B. Suppose that bottom or zero line represents an elevation of 55 m and the successive thicker line represents 60 m, 65 m, 70 m etc. Place the tracing paper so that the point A is on the 5th line. Now, turn a tracing paper until the point B is on the 27th line from the zero line (or on the 7th line from the second thicker line). The intersections of 1st and 2nd thicker line representing elevations 60 m and 65 m and the line AB will provide the positions of the points on the 60 m and 65 m contours, respectively. These positions are pricked by on to the plan.