USCG Exam PracticePlotting with variation and deviation (T-V-M-D-C)
When measuring distance on a Mercator chart using dividers, against which scale must the measurement be taken?
- A. The longitude scale at the working longitude of the line
- B. The latitude scale at the equator for all measurements
- C. The latitude scale at the top or bottom border of the chart
- ✓ The latitude scale at the working latitude of the lineCorrect
Why D is correct
One minute of latitude equals one nautical mile, so distance is measured on the latitude scale alongside the vessel's working latitude. The longitude scale must never be used for distance measurement, and the latitude scale at the equator or chart border would introduce error at other latitudes. (Bowditch Ch. 4 §405, §402)