Navigation rests substantially on determining positions. The sea surface is almost two-dimensional; a position is thus given by two coordinates, so that two independent measurements are needed.

At sea distances are traditionally measured using the sextant, essentially a goniometer. If the height of a marked object on shore is known, for example from the sea chart, the height angle is used to determine ones own distance from the object.

The traditional astronomical navigation at sea, for which the sextant is also used, reaches an accuracy of a few kilometres. It is thus feasible for a coarse assessment of position only.