Module 10 - Chartwork
Waypoints and GPS Navigation
What you will learn
Use GNSS waypoints and route displays as navigation evidence while checking route geometry, source quality and independent backup.
A waypoint is only a coordinate. It becomes useful when the navigator checks how it was entered, what lies between waypoints, and what independent information remains if the primary system or signal fails.
GNSS receivers, often described as GPS, can provide position, course over ground and speed over ground. A plotter can then show bearing and distance to a waypoint and cross-track error from a route leg. These are powerful aids, but a route line between two safe-looking points is not itself a checked safe route: inspect the whole leg at a suitable scale for hazards, depth, chart notes and chart currency.
A plausible display can still be wrong because of an incorrect waypoint, wrong chart datum, outdated or unsuitable chart data, user settings, loss of integrity, jamming, spoofing, power failure or damaged equipment. Cross-check position and trend against available visual information, depth, radar, expected tidal effect and the vessel's recent DR or EP rather than waiting for an obvious alarm.
The safe backup depends on the vessel and passage. RIN guidance favours a means of navigation independent of the primary system and power supply. A current paper chart kept in use can be one solution; an independently powered second electronic system may be another when its data and positioning vulnerabilities are understood. Two apps on one device, using the same GNSS and battery, are not independent.
Check the route, not only the destination
A waypoint imported from a message appears in the correct harbour, but the automatic route crosses a charted drying area.
1. Verify the coordinate
Check latitude, longitude, datum, name and source against the intended location.
2. Inspect every route leg
Zoom to a useful scale and check depths, hazards, clearances, notes and the approach rather than trusting the overview line.
3. Set monitoring cues
Choose cross-track, depth, position and waypoint checks that leave time to act, while retaining an independent backup method.
Sense check: A correct destination paired with an unsafe line between waypoints is still an unsafe route.
Common mistake or limitation
- Zooming out can hide hazards or simplify detail; a clear screen is not evidence of clear water.
- A second app sharing the same device, battery and GNSS is not a fully independent backup.
Recap
- GNSS and plotters are valuable sources, not infallible authorities.
- Validate coordinates and inspect the complete route at a suitable scale.
- Plan a genuinely independent way to continue safely after primary-system failure.
Optional quick check
Section 6 of 7
Which arrangement is the strongest independent backup to a tablet used as the primary plotter?
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.