Navigation and Position Fixing
Fixing and compass work
This domain tests practical position fixing, chart datum, tidal height, compass error and the limitations of GNSS/ECDIS. The examiner wants evidence that you can produce a defensible fix, not just read a screen.
- Fix at intervals suited to speed and proximity of danger; use two or more independent position lines and label time and log reading.
- Compass conversion follows C D M V T: Compass, Deviation, Magnetic, Variation, True; easterly errors are added left-to-right.
- Check the echo sounder against charted depth plus height of tide; allow for squat and under-keel clearance.
- GNSS: confirm datum/transform, HDOP, RAIM and antenna offset before plotting.
- A cocked hat larger than the expected fixing error should prompt a new fix, not a guessed centre.
Worked example
True bearing 045°T, variation 6°W, deviation 2°E. Magnetic = True − variation east; variation 6°W is −6°E, so 045 − (−6) = 051°M. Compass = Magnetic − deviation east = 051 − 2 = 049°C. Check: 049°C + 2°E = 051°M; 051°M − 6° = 045°T. Helm order: steer 049.
Traps
Applying variation in the wrong direction, or using ship's head instead of the bearing line. Plotting a GNSS position on a chart of a different datum without transformation. Treating charted depth as true depth and ignoring tide, squat or UKC. Forgetting the gyro error sign when converting a gyro bearing to true.