By Arithmetic Calculations

This is extremely accurate method but time consuming also. Position of contours points among guide points are located by arithmetic calculations. For example, A and B are two ground points having their elevations as 53.65 m, and 56.85 m respectively. The distance among these points is 15 m and let the contour interval is 1 m. Let among A and B the contours of 54 m, 55 m and 56 m are to be located. The contours could be located as follows:

Difference of level among A and B = 56.85 - 53.65 = 3.20 m.

Difference of level among A and the 54 m contour point

= 54 - 53.65 = 0.35 m.

Thus, distance of 54 m contour point from A = (0.35/3.2) × 15 = 1.64 m. Same, the distances of 55 m and 56 m contour points from A could be computed as 6.33 m and 11.01 m, correspondingly. These distances could be plotted to the scale on the map.