Chart Navigation & Plotting · Chart Distance — Latitude Scale
On a Mercator chart, distance should always be measured using the:
- ALongitude scale at the top or bottom
- BCompass rose
- CLatitude scale at the side, adjacent to the area being measured✓ Correct
- DBar scale only at the equator
Explanation
One minute of latitude equals one nautical mile, so the side (latitude) scale is used, taken at the same latitude as the measurement because the Mercator latitude scale expands toward the poles. The longitude scale must never be used for distance.
Authority: Bowditch (Pub. No. 9); chart use
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