Module 10 - Chartwork
Position Fixing
What you will learn
Judge a coastal position fix by object identity, observation timing, geometry and independent agreement instead of trusting its symbol or cocked-hat size.
A fix comes from observations, but observations can share mistakes. The navigator needs useful geometry, prompt timing and another reasonableness check, especially near danger.
A bearing to a correctly identified charted object creates a line of position. Two suitably separated lines can intersect; a third can expose disagreement. A transit may provide a strong line because two charted features visibly align. Radar range, depth-contour evidence and a checked GNSS position can contribute, but each method has its own identification, datum, instrument and plotting limits.
Three bearing lines often form a cocked hat instead of one point. It displays inconsistency between the observations or plot; it does not prove that the vessel lies inside. A wrong compass correction, biased instrument or mistaken feature can displace every line in a similar direction. Near danger, work from the dangerous side of the uncertainty and obtain a genuinely different check.
From a moving vessel, normally take the slowly changing bearing near ahead or astern first and the rapidly changing bearing near abeam last, then use the final observation time. This corrects a common but unsafe shortcut: taking the abeam bearing first lets the fastest-changing line become the stalest by the time the sequence ends.
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.
Respond to a neat but displaced cocked hat
Three bearings create a small triangle west of the route. Shoal water lies east, and every bearing used the same hand-bearing compass correction.
1. Inspect shared error
Check object identity, compass error, bearing direction, time and plot orientation before treating the small triangle as accurate.
2. Use the danger-side case
Allow for the vessel being east of the triangle or outside it, not automatically at its centre.
3. Change the evidence source
Seek a transit, radar range, depth trend or independently checked electronic position as appropriate.
Sense check: Three precise-looking lines sharing one wrong correction can agree with one another and still place the vessel on the wrong side of danger.
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 final observation time.
Recap
- Object identity, timing and geometry govern a visual fix.
- A cocked hat is evidence of disagreement, not a containment guarantee.
- Use a different observation method to challenge a shared error.
Optional quick check
Section 3 of 13
Which observation order best limits movement error during a quick three-bearing fix?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — use of electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- RIN guidance for safe use of electronic navigation on leisure vessels
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
Continue studying Chartwork
This public lesson is part of Revision Module 10. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.