In relative plotting the movement of the observer's ship is ignored. The first bearing B1 and distance a1 are laid off from a chosen origin and after a determined time-interval the second bearing B2 and distance a2 are plotted from the same point. By connecting these two points the relative course of the observed ship is obtained, which enables the observer to tell whether the other ship will cross ahead or astern, what her distance off will be, and when the intersection will occur.