Chart Navigation & Plotting · Chart Basics — Measuring Distance
On a Mercator chart, nautical miles are measured using the:
- ACompass rose radius
- BLongitude scale along the top
- CBar (graphic) scale only at the equator
- DLatitude scale at the side, at the same latitude as the line being measured✓ Correct
Explanation
One minute of latitude equals one nautical mile, but the latitude scale stretches toward the poles on a Mercator chart, so distance is taken from the side scale at the same latitude as the measured line. The longitude scale is never used for distance.
Authority: Bowditch (Pub. No. 9); Chart No. 1
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