Graphical Method

Various graphical methods are suggested to solve the three-point problem. Therefore, the Bessel's solution is the most generally used method in practice being the simplest. The Bessel's solution could be defined in the following steps:

(a)        The plane table is levelled and set up at new station O. The alidade is placed along known line (say ba on the map) and table is rotated until A is sighted along with 'a' pointing towards A as shown in Figure 8(a), clamp the table and sight C with alidade centered on b, draw a line x-x along alidade edge.

(b)       The alidade is now placed along ab and table turned to bisect B with b towards B as in Figure 8(b). Clamp the table and centre the alidade at a, bisect C through drawing the ray aC intersecting the previously drawn ray x-x at point c′ (say). Join cc′.

(c)        Alidade is now placed along c′c as in Figure 8(c) and table turned till C is bisected and clamped within new position. The table is right oriented.

(d)       The alidade is centered at b and B is bisected. Illustrate the ray to intersect cc′ in o. same, if alidade is pivoted about a and A is sighted, the ray will pass by o if the process is accurate.

Any minor error is corrected therefore.