To estimate distance traveled and to keep track of his position during a voyage, a navigator uses what he calls "dragging" or "drags." It involves using an island other than his destination as a point of reference.
For example, on a voyage from Puluwat to Tol, in the Truk Islands, the island of Pisaras serves as reference. The course from Puluwat to Tol is almost directly east on the rising of Altair. Pisaras lies, out of sight, 120 miles (190 kilometers) northeast of Puluwat on the rising of Vega and a like distance northwest of Tol on the setting of Vega. As the voyager goes the 150 miles (245 kilometers) from Puluwat to Tol, Pisaras is "dragged" from the rising of Vega through the rising of Cassiopeia, the rising of the main star in Ursa Major, the rising of Kochab in Ursa Minor, Polaris, and on through their respective settings to the setting of Vega on arrival at Tol. Each next compass position to which Pisaras is "dragged" begins a new leg or "drag" of the journey.
The whole journey in this case is thus divided into eight "drags." Estimating the headway he is making, a navigator keeps track of his progress from one "drag" to another. As changing conditions affect his progress, he need adjust his reckoning only from one "drag" to the next. This way it is easier to keep overall track of progress made and expectation of landfall.
Every course between islands has an island or seamark of reference that serves to divide the journey into "drags." Ideally, the end of the first "drag," the "drag of visibility," corresponds with the point at which the island of departure ceases to be visible; and the end of the second, the "drag of birds," corresponds with the most distant point at which land-based birds feed at sea. Similarly, the next to last is the one at which land-based birds again appear, and the last one begins when the island of destination becomes visible.
Imaginary places can serve as points of reference for "dragging"
as well as real ones, since real places that serve as such points
are always too far away to be visible. For the trip north from the
Carolines to Guam and Saipan, there are no conveniently located islands.
Here "ghost islands" are used as reference.