It requires that the navigator know three things, as shown in the illustration below: (1) his ship’s starting point, (2) its speed, and (3) its heading (direction of travel).
See eeldas, et meresõitja teab kolme asja, nagu ilmneb alltoodud illustratsioonist: (1) laevareisi lähtepunkti, (2) laeva kiirust ja (3) kurssi.