USCG Exam PracticeDead reckoning — DR plot maintenance
A navigator is maintaining a DR plot in open water using only course steered and speed through the water. No external fix has been obtained for 4 hours. Which combination of factors from the source material BEST explains why this DR plot may contain significant positional error, and what action is required?
- A. Electronic navigation is the principal real-time reference and renders DR plot maintenance unnecessary on modern vessels
- B. Piloting is unavailable in open water, so the navigator must switch to celestial navigation; no DR plot maintenance is required
- ✓ Cumulative errors from instrument, observation, plotting, and computation sources compound over time, and the DR assumption that the vessel has held the planned course ignores set and drift; the plot must be confirmed against external fixes whenever possibleCorrect
- D. The running fix method should replace DR after two hours because simultaneous LOPs are more accurate; the DR plot should be abandoned
Why C is correct
Bowditch Ch. 23 §2301 identifies multiple compounding error sources in navigation, and Ch. 23 §2302 notes that set and drift are not captured in the DR assumption. Bowditch Ch. 1 §102 explicitly states DR accuracy degrades over time and must be confirmed by external fixes whenever possible. Option D misapplies Bowditch Ch. 1 §101, which describes electronic navigation as the principal real-time reference but not a replacement for DR cross-checking.