Module 10 - Chartwork
Position Fixing
What you will learn
Assess a visual or transferred fix by the quality, timing and geometry of its observations rather than treating the plotted symbol as proof.
A fix is derived from observations, but every observation and plot has error. Good fixing combines identifiable features, useful geometry, prompt observations and an independent reasonableness check.
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.
Worked example
At 1015 you take the first bearing to a lighthouse. At 1030 you take a second bearing to the same lighthouse after making an estimated 1.2 M over the ground.
- 1Plot the 1015 line of position through the correctly identified lighthouse.
- 2Transfer that line parallel to itself by the estimated 1.2 M movement to 1030.
- 3Plot the 1030 bearing; its intersection with the transferred line is the running fix.
- 4Label the result and remember that an error in the 1.2 M movement shifts the running fix.
Sense check: The transferred line should move in the same direction and by the same estimated ground displacement as the vessel.
Treat the cocked hat as a warning
Three lines form a small triangle just outside a planned track, while shoal water lies close to its eastern side.
1. Inspect the observations
Check object identity, compass correction, plotting direction, time and whether the features gave useful geometry.
2. Use the dangerous-side assumption
Do not select the centre automatically; allow for the vessel being nearer the shoal or even outside the triangle.
3. Cross-check promptly
Use depth, a transit, a fresh bearing, radar range or an independently powered electronic source as appropriate.
Sense check: A small cocked hat can still be displaced by a shared systematic error, so size alone does not prove accuracy.
Common mistake or limitation
- The centre of a cocked hat is not automatically the vessel's true position.
- Taking the rapidly changing abeam bearing first makes it stale by the time the final bearing is observed.
Recap
- A fix is only as good as its observations, timing and geometry.
- A cocked hat displays inconsistency, not a guaranteed containment area.
- Transferred position lines add movement-estimate uncertainty.
Optional quick check
Section 3 of 7
In which order should quick visual bearings normally be taken from a moving vessel?
Continue studying Chartwork
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