At 2245 your GPS position is LAT 41°01.75'N, LONG 72°48.40'W. You are steering course 086° per standard magnetic compass at a speed of 6.0 knots. At 2400 you fix your position by plotting several compass bearings on nearby known fixed objects. These result in a position of LAT 41°04.20'N, LONG 72°38.85'W. What were your set and drift?
• Dead reckoning (DR) track vs. actual track over ground • How to compute difference between DR position and fix to find set and drift • Converting distance run and distance off track into a course (direction) and speed (knots) of current
• First, plot the DR position at 2400 assuming you actually made good your steered course and speed with no current. Then compare this to the fixed (observed) position to see how far and in what direction you were pushed. • Measure the direction (true bearing) from the DR position to the fix. That direction is the set of the current. Then measure the distance between DR and fix and divide by the elapsed time to find the drift. • Be careful about time: how many minutes passed between 2245 and 2400, and how do you convert this fraction of an hour when you compute drift in knots?
• Confirm the elapsed time between the two positions in hours (not minutes). • Check that your DR position at 2400 is correctly computed from the initial position using the course steered and speed through the water. • Verify that the set is expressed as the true direction from DR to fix, and that the drift (speed of the current) matches one of the answer choices when rounded appropriately.
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