Running Fix

A position fix obtained by advancing an earlier line of position to the time of a later observation.

It is commonly used when the same charted object is observed twice from a moving vessel, but observations of different objects or other lines of position can also be combined. The earlier line is transferred by the vessel’s estimated course and distance made good between observation times and intersected with the later line.

Accuracy depends on both observations and on estimated movement, including course, speed, leeway and tidal stream. A running fix is therefore less independent than simultaneous lines of position and should be labelled with times and checked against depth, GNSS or other available information.