If an observer sees two object in line ashore, ie beacons, then he must lie somewhere on the extended line joining them. This provides an excellent position line, provided the distance between the observer and the nearer of the two objects is no more than three times the distance between the objects themselves.

Transit.

A transit can also be used to check the accuracy of the compass, ie if there is any error present. When the compass (gyro or magnetic) bearing is observed when two charted objects are in line and compared with the true or magnetic bearing between the two objects on the chart, the difference is the gyro error/deviation of the gyro/magnetic compasses respectively.