Resection after Orientation by Back Sighting

Given figure shows the scheme of resection by back sighting after orientation. From station A, the position of B is plotted as 'b' and ray has been taken to station P as ap′. Then plane table is set at P and oriented by back sighting A, line AP is not measured but by back sighting the position of P is obtained on the paper by back sighting.

                               Resection after back sighting