Set and drift

Set is the compass direction toward which a current flows, and drift is its speed, usually in knots; together they form the current vector applied to a dead-reckoning position to obtain an estimated position. The set and drift over a leg can be recovered by comparing the DR position with an observed fix at the same time: the displacement between them, divided by the elapsed time, gives the average current experienced. The same vector triangle is used in advance to plan the course to steer that will make good a desired ground track.