Chart Navigation & Plotting · Chart Basics — Measuring Distance
On a Mercator chart, distance should be measured using:
- AAny convenient scale, since all are equal
- BThe longitude scale along the top or bottom
- CThe compass rose diameter
- DThe latitude scale at the side of the chart, at the same latitude as the area being measured✓ Correct
Explanation
On a Mercator chart the latitude scale expands with latitude, so 1' of latitude = 1 nm only at the latitude in question. Always measure distance on the latitude (side) scale adjacent to the area being measured; the longitude scale must never be used for distance.
Authority: Bowditch (Pub. No. 9); Chart No. 1; Mercator chart conventions
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