A transit is taken by sighting two objects in line. If the objects can be located on the chart, the observer is obviously somewhere on an extension of the line passing through them, and such knowledge is invaluable in pilotage. Furthermore, if the bearing of such a transit is observed with the compass, the reading can be compared with the magnetic bearing derived from the chart and any compass deviation can be deduced. A practical navigator will always be on the look-out for pairs of objects which provide good transits, and will frequently use them to check his compass.