Position Fixing

A bearing to a correctly identified charted object creates a line of position: the vessel lies somewhere on that line at the observation time. Two suitably separated lines may intersect at a point; a third provides a useful check. A transit can give a strong position line because two charted objects are visibly aligned. A checked GNSS position is another observation source, but its display still needs the correct chart, datum and interpretation.

Three bearing lines often form a small triangle called a cocked hat rather than meeting at one point. The triangle shows disagreement between the observations or plotting; it does not prove the vessel lies inside it. A shared error, such as wrong compass correction or mistaken feature identification, can move every line in the same direction. Near danger, use the conservative side of the uncertainty and obtain another check.

When taking several visual bearings from a moving vessel, take the slowly changing bearing near ahead or astern first and the rapidly changing bearing near abeam last, then use the final observation time for the fix. A transferred position line moves an earlier observation to a later time using the best available estimate of movement between them. Its result inherits uncertainty from both the observation and the transferred movement.