USCG Exam PracticeThree-bearing fix and cocked-hat triangle
A vessel is transiting a narrow channel. The navigator has only one charted object visible and cannot obtain a simultaneous two-object fix. The navigator takes a bearing of 035°T to the object at 1200, runs 1.5 miles on course 090°T, then takes a second bearing of 310°T to the same object at 1212. The navigator advances the 1200 LOP 1.5 miles along the 090°T DR track and plots the running fix. Compared to a simultaneous three-bearing fix, this running fix is considered less reliable primarily because:
- ✓ It assumes course and speed were maintained exactly between the two observations, and any deviation introduces error into the advanced LOPCorrect
- B. A single charted object cannot produce a valid line of position under any circumstances
- C. The two bearings must differ by exactly 90 degrees for the running fix to be valid
- D. The advanced LOP must be taken to a different object than the second bearing to produce a valid intersection
Why A is correct
A running fix is less reliable than a simultaneous fix because advancing the first LOP along the DR track assumes the vessel held the stated course and speed exactly. Any deviation in course or speed between observations introduces error into the advanced LOP and therefore into the running fix position. (Bowditch Ch. 7 §708, Bowditch Ch. 7 §702)