Your present position is LAT 41°05.5'N, LONG 72°38.0'W. Assuming that there is no set and drift, what course must you steer per standard magnetic compass (psc) to arrive at a position 2 miles due west of Twenty-Eight Foot Shoal Lighted Buoy (LAT 41° 09.3' N, LONG 72° 30.5' W)?
• Plotting a course between two positions on a Mercator chart using latitude and longitude • Finding the difference in latitude and difference in longitude and using them to determine true course • Converting true course to compass course (psc) using variation and deviation
• First, think about how you would plot both positions on the chart and how to determine the bearing from your starting point to a point that is 2 miles due west of the buoy, not to the buoy itself. • Consider how the change in longitude at about 41°N converts to minutes of arc and horizontal distance; how does that help you get the course between the two points? • After you find the true course, ask yourself: what corrections (east or west) must you apply for magnetic variation and deviation to end up with a standard magnetic compass (psc) course?
• Be sure you applied the 2 miles due west offset from Twenty-Eight Foot Shoal Lighted Buoy before drawing your course line. • Verify that your computed difference in latitude and departure are consistent with the scale of the chart at 41°N. • Double-check the sequence of course corrections: True ➝ Magnetic (apply variation) ➝ Compass (apply deviation) ➝ Standard compass (psc), paying attention to the algebraic signs of each correction.
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