Intersection

In place of one ground station O₁, as in radial method, two ground stations O₁ and O₂ are chosen on ground, like that all significant features of area to be surveyed are sightable from both stations. The line merging instrument station O₁ and O₂ is called base line. It is the only distance which is required to be measured linearly on ground. Along with plane table positioned at one station (say O₁) a point is transferred on sheet as O₁ as in last method. With alidade pivoted at O₁ various survey points A, B, C ... are sighted and radial lines O₁a, O₁b, O₁c are drawn. Further the plane table is shifted and positioned at O₂. Within alidade pivoted at O₂, survey points A, B, C ... are sighted again and radial line O₂A, O2B, O₂C ... are drawn on sheet. The intersection of radial lines, e.g. (O₁a and O₂a) will provide the location of A on sheet as 'a' and so on, without making any linear measurement as display in Figure 4.

                                                  Figure: Method of Intersection

This method is commonly preferred for plotting the details of ground, objects, that are far away or hard to access, rivers etc., and the survey stations that could be subsequently used as instrument stations. It is particularly meaningful in rough and uneven regions while accurate linear measurements are tedious, or hard or even impossible in a few cases.