A vessel at LAT 32° 14.7' N, LONG 66° 28.9' W, heads for a destination at LAT 36° 58.7' N, LONG 75° 42.2' W. Determine the true course by Mercator sailing.
• Mercator sailing: relationship between difference of longitude, meridional parts, and course • Determining the quadrant of the course from starting and destination lat/long • Using difference of latitude (ΔLat) and difference of longitude (ΔLong) correctly (east vs west, north vs south)
• From the two positions, first decide: is the destination north/south and east/west of the departure? That tells you which general direction (NE, SE, SW, NW) the course should lie in. • Recall the Mercator sailing triangle: how are ΔLong, meridional difference of latitude (ΔM), and course related? Which trig function links them? • After you estimate the quadrant, compare your calculated course with the answer choices and eliminate any that fall in the wrong quadrant.
• Verify which latitude is greater to confirm north/south relationship. • Verify which longitude is greater, remembering that in western longitudes a larger degree number is farther west. • Check that you used meridional parts (M) for each latitude (from tables or calculator) and took the difference of meridional parts (ΔM), not plain difference of latitude, in the Mercator course formula.
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