Module 10 - Chartwork
Advanced Scenario Check: Chartwork
A fictional yacht is approaching a headland in reduced visibility. Its chartplotter and tablet show the same GNSS position, electronic chart and copied route. A correctly identified radar range and a timed visual position line support a fix displaced towards danger, while checked echo-sounder depth is trending 0.6 m shallower than the plan expected.
Do not average the electronic position and the observation-based fix. They are not repeated measurements of equal independence and quality. Trace the GNSS, chart, route and power dependencies; confirm radar range scale and feature identity; check the bearing time and reference; and compare depth only after accounting for transducer offset, tide, squat, seabed shape and chart datum.
The shallower depth is not an exact horizontal fix, but it is evidence that challenges the displayed navigation picture. The observation-based fix also retains identification, timing and plotting uncertainty. Until the conflict is resolved, preserve the danger-side case, reduce exposure as conditions require and use the briefed sea-room, holding or diversion option rather than steering towards an averaged point.
This scenario rehearses evidence control. It cannot prescribe the live course, speed, passing side or clearance for a real yacht; those decisions depend on current conditions, vessel manoeuvrability, traffic, tide, charted dangers and the skipper's assessment.
| Evidence | What it does not prove | Safe use |
|---|---|---|
| Two displays share GNSS, chart and route | Independent position or route confirmation | Treat agreement as one source path |
| Radar range and timed visual line | An error-free exact fix | Verify identity, timing, reference and plotting geometry |
| Depth trend 0.6 m shallower | One precise horizontal position or one cause | Challenge clearance and position after offset, tide, squat, seabed and datum checks |
| Averaged electronic and observed positions | A defensible compromise | Do not average; resolve dependencies and protect the danger-side case |
Optional quick check
Section 13 of 13
The plotter and tablet share GNSS, chart and route data, while radar, a visual line and depth trend disagree. What is the strongest next step?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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