A position obtained by advancing a known fix using course and speed while applying estimated leeway and current allowances.
Quick facts
Regulation
STCW Section A-VIII/2
In practice
The OOW plots an EP whenever a proper fix cannot be obtained—in thick fog, at night beyond the range of coastal lights, or when GPS is unavailable. The EP deteriorates in reliability as time elapses since the last fix, particularly in strong tidal streams or where current direction is uncertain. IMO guidance on watchkeeping (STCW Section A-VIII/2) requires officers to maintain a running plot and to be aware at all times of the margin of uncertainty surrounding the estimated position, taking whatever cautionary action is appropriate to the proximity of hazards.
Regulatory detail & full definition
An estimated position (EP) is the best assessment of a vessel's current location obtained by advancing the last known fix using the course steered, speed through the water, and applying estimated allowances for leeway, tidal streams, and ocean currents. Bowditch (American Practical Navigator) distinguishes the EP from the dead-reckoning position (DR) precisely by the inclusion of these additional factors; the EP is therefore regarded as more reliable. On the chart or ECDIS plot, an EP is conventionally marked with a square symbol, whereas a DR position is marked with a semi-circle, and a confirmed fix with a circle.
The OOW plots an EP whenever a proper fix cannot be obtained—in thick fog, at night beyond the range of coastal lights, or when GPS is unavailable. The EP deteriorates in reliability as time elapses since the last fix, particularly in strong tidal streams or where current direction is uncertain. IMO guidance on watchkeeping (STCW Section A-VIII/2) requires officers to maintain a running plot and to be aware at all times of the margin of uncertainty surrounding the estimated position, taking whatever cautionary action is appropriate to the proximity of hazards.