Module 10 - Chartwork
Waypoints and GPS Navigation
What you will learn
Validate an electronic waypoint and route, interpret geometry quality without treating it as integrity, and retain a genuinely independent navigation method.
A waypoint is a coordinate, not a safety certificate. The route, chart data, receiver solution, settings, power and independent backup all need separate checks.
GNSS receivers can provide position, COG and SOG; plotters can show bearing and distance to a waypoint and XTE from an active route leg. Before use, verify the coordinate, datum, route direction, chart source and every leg at a scale that reveals relevant dangers, depths, clearances and notes. A correct destination joined by an unchecked line can still create an unsafe route.
A plausible display may be wrong through entry error, unsuitable or outdated chart data, hidden scale, sensor selection, signal blockage, receiver fault, jamming or spoofing. Horizontal dilution of precision describes how satellite geometry scales horizontal ranging error in the solution; a low HDOP is favourable geometry, not proof that the position, chart or signal is accurate or free from interference.
Cross-check position and trend before a discrepancy becomes critical. Visual bearings, transits, radar ranges, depth trends and the recent DR or EP can provide different evidence. A backup should avoid the primary system's important common failure modes. Two apps on one tablet or two receivers using the same compromised signal and chart source may detect some device faults but do not provide complete independence.
| Failure or mistake | Possible cross-check | Shared-failure warning |
|---|---|---|
| Wrong waypoint or route leg | Visual geometry and chart inspection | Copying the same route to another app repeats it |
| GNSS position anomaly | Transit, radar, depth and DR/EP trend | Another receiver may share the same signal error |
| Primary device or power loss | Independently powered maintained method | Two apps on one tablet share device and battery |
Investigate a smooth but impossible COG
The plotter shows the yacht following the route perfectly, but visual transits and depth both place it east of the displayed track.
1. Protect the vessel first
Use the dangerous-side evidence, reduce exposure and avoid steering solely to recover the displayed line.
2. Check common inputs
Review waypoint entry, chart source and scale, datum, selected position sensor, antenna status and recent warnings.
3. Continue independently
Use the maintained alternative position and route method until the discrepancy is understood and the primary system is trustworthy again.
Sense check: Agreement between multiple displays fed by one GNSS source is one repeated answer, not multiple independent observations.
Common mistake or limitation
- Low HDOP cannot rule out a wrong waypoint, chart error, jamming, spoofing or shared sensor failure.
- A second app on the same device and battery is not a fully independent backup.
Recap
- A coordinate and route line both require validation.
- Geometry quality and solution integrity are different questions.
- Plan an independent way to continue safely after primary-system failure.
Optional quick check
Section 9 of 11
What does a low HDOP allow a navigator to conclude on its own?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
- 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.