USCG Exam PracticePlot true course and distance between two charted points
When measuring distance on a Mercator chart using dividers, you should measure against which scale?
- A. The longitude scale along the top or bottom of the chart
- B. The compass rose diameter
- ✓ The latitude scale at the working latitude of the lineCorrect
- D. Any convenient scale, since the chart is conformal
Why C is correct
One minute of latitude equals one nautical mile, and distance must be measured on the latitude scale at the working latitude of the line because meridian length varies with latitude on a Mercator chart. The longitude scale must never be used for distance measurement. (Bowditch Ch. 4 §405, §402)