Fix

A fix is a confirmed position of the vessel established by simultaneous or near-simultaneous observation of two or more independent position lines. Each position line is derived from a bearing, range, or depth sounding; where they intersect, the vessel is located. Bowditch (American Practical Navigator) distinguishes a fix from a running fix and from an estimated position by the simultaneity of observations and the independence of the data sources. The Admiralty Manual of Navigation specifies that for a position to be called a fix, the position lines must ideally cross at angles close to 90° (for two lines) or at 60° intervals (for three lines) to minimise the cocked-hat triangle and maximise accuracy. The OOW obtains fixes at regular intervals determined by speed, proximity to hazards, and the precision required. In open ocean, hourly GPS fixes may suffice; in confined channels or port approaches, fixes every few minutes are essential, combined with continuous radar monitoring. SOLAS Chapter V and STCW Section A-VIII/2 both emphasise continuous position monitoring as a core OOW obligation. When three position lines produce a cocked-hat triangle rather than a single point, the OOW should place the ship at the corner of the triangle closest to the danger as the conservative safe assumption.