Chart Navigation & Plotting · Piloting — Vertical Sextant Angle
You measure a vertical sextant angle of 0°30' to the top of a lighthouse charted as 113 feet high. Using distance(NM) = 0.565 × height(ft) ÷ angle(minutes), how far off is the lighthouse?
- A0.53 NM
- B1.07 NM
- C3.39 NM
- D2.13 NM✓ Correct
Explanation
0°30' = 30 minutes of arc. Distance = 0.565 × 113 ÷ 30 = 63.845 ÷ 30 = 2.13 NM. The vertical-sextant-angle method gives an accurate distance off a charted object of known height.
Authority: Bowditch (Pub. No. 9), Distance by Vertical Angle
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