At 0620 your GPS position is LAT 41°01.8'N, LONG 72°48.40'W. You are steering course 274° per standard magnetic compass at a speed of 10 knots. At 0735 you fix your position by plotting several compass bearings on nearby known fixed objects. These result in a position of LAT 40°59.50'N, LONG 73°06.50'W. What were your set and drift?
• Dead reckoning (DR) track from course and speed over the time run • Difference between intended track (DR position) and fix (position made good) to find set and drift • Converting distance in nautical miles and time in hours to drift (knots)
• First, compute how far you should have traveled from 0620 to 0735 at 10 knots and plot that DR position from the starting GPS position. Where does that DR point lie? • Next, draw a vector from the DR position to the 0735 fix. In which true direction does that vector point, and how long is it in nautical miles? • Finally, relate the length of that vector to the elapsed time to determine the speed of the current. Which answer choice matches that direction and speed?
• Verify the elapsed time between 0620 and 0735 and convert it correctly to hours (decimal). • Carefully measure the distance and direction from DR position to the fix; remember that set is the direction toward which the current flows (°T). • Double-check that your computed drift (nm per hour) matches one of the listed speeds in knots before picking an answer.
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