USCG Exam PracticePlot true course and distance between two charted points
A vessel is steaming on a course of 270°T at 10 knots. The navigator needs to measure the distance run in 2 hours on the chart. At what latitude should the dividers be set against the latitude scale?
- A. At the equator, where the scale is most uniform
- B. At the latitude of the departure point only
- ✓ At the working latitude of the line being measuredCorrect
- D. At any latitude, since the Mercator scale is constant throughout
Why C is correct
On a Mercator chart, meridian length varies with latitude, so distance must always be measured on the latitude scale at the working latitude of the line. Using the equatorial scale or any other latitude would introduce error. (Bowditch Ch. 4 §402, §406)