Module 10 - Chartwork
Advanced Scenario Check: Chartwork
What you will learn
Diagnose shared-source agreement and preserve the safe side when electronic position, independent observations and depth trend conflict.
Several agreeing screens can still be one evidence path. The command task is to trace dependencies, avoid averaging unlike observations and keep the vessel in controllable water while the navigation picture is rebuilt.
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.
Worked example
Classify the four displayed or observed inputs before deciding whether the original route can remain open.
- 1Chartplotter and tablet position: one GNSS solution repeated on two displays, not independent corroboration.
- 2Copied electronic chart and route: shared user-entered and chart-data dependencies that can repeat the same waypoint or portrayal error.
- 3Radar range plus timed visual line: observation-based evidence with different failure modes, subject to feature identity, timing, reference and plotting checks.
- 4Depth 0.6 m shallower than expected: a material trend that challenges the plan after instrument offset, tide, squat, seabed and datum are checked; it does not identify one exact position by itself.
- 5Decision: keep the original route closed, preserve the safe side and rebuild position and clearance from evidence with genuinely different failure modes.
Sense check: Averaging a shared GNSS position with a radar-and-visual fix would produce a tidy point without proving which source is wrong or how much safe-water margin remains.
| 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 |
Run a shared-source challenge
The tablet is described as the backup because it has its own battery, but it receives the same GNSS position and copied route as the main plotter.
1. Trace dependencies
List position sensor, chart edition and updates, route source, power, antenna and network path for each display.
2. Check observations
Verify radar scale and target identity, visual-line time and reference, echo-sounder offset and the applicable tide and chart datum.
3. Protect the danger side
Use the conservative position and clearance case while keeping sea room, a holding area or diversion available.
4. Define reopening evidence
Require a coherent fix, depth trend and track history from genuinely different failure modes before the original route is reconsidered.
Sense check: Independent battery power may protect against one electrical failure, but it does not make shared GNSS, chart or route data independent.
Common mistake or limitation
- Two displays are not independent when they repeat the same GNSS, chart or route data.
- A depth discrepancy can challenge a position but does not by itself prove one precise location or one cause.
- Averaging unlike or dependent evidence hides conflict rather than resolving it.
- An electronic backup must be usable for the intended failure and should not share every critical dependency it is meant to cover.
Recap
- Count data paths and failure modes, not screens.
- Verify observation identity, timing, reference and instrument context.
- Do not average conflicting dependent and independent evidence.
- Protect the danger-side case and a controllable fallback until the picture is coherent.
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 2 September 2026 - Compass Revision editorial review.
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.