At 0817 your GPS indicates a position of LAT 37°01.6'N, LONG 75°31.7'W. You are on course 182° per standard magnetic compass at a speed of 9.2 knots. At 0913 your GPS indicates your position at LAT 36°52.3'N, LONG 75°30.8'W. What were the set and drift?
• Set and drift: set is the direction the current pushes you (from your intended track), drift is the speed of that current in knots • Difference between dead reckoning (DR) position and fix (your GPS position) over the same time interval • Converting time run at given speed into distance, and comparing that to the actual distance made good
• First, if you had no current, where should you be at 0913 based on your course and speed from the 0817 position? Plot or calculate that DR position. • Next, compare the DR position to the actual 0913 GPS position: in what true direction did the current move you, and how far? • From the distance between DR and fix and the elapsed time, what is the speed of that current, and how does that compare with the answer choices?
• Carefully compute the elapsed time between 0817 and 0913 in hours and minutes, then convert to hours as a decimal before using it in speed/distance formulas. • Make sure you are working in true directions when labeling the set from DR to fix (do not use the magnetic course directly for set). • Double-check your distance measurement (in nautical miles) between DR and fix, then use ( \text{Drift} = \frac{\text{Distance (NM)}}{\text{Time (hours)}} ) and compare it precisely to the listed drifts.
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