At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1031 your position is LAT 36°59.5'N, LONG 75°44.9'W. What was the set and drift?
• How to compute DR (dead reckoning) position from course and speed over a known time interval • Vector difference between DR position and observed (fix) position gives set and drift • Set is the direction toward which the current pushes; drift is the speed of that current in knots
• First, calculate where you should be at 1031 if there were no current, using your 0919 position, course 270°T, and speed 8.7 knots • Then, compare that DR position to your actual 1031 fix to find the direction and distance the current moved you • Convert that distance and direction into a current vector: determine its true bearing (set) and divide distance by elapsed time to find the drift in knots
• Convert the elapsed time from 0919 to 1031 into hours accurately before computing distance run • Be sure your latitude and longitude differences have the correct signs and directions (N/S and E/W) when plotting or calculating • Confirm that your final current direction is expressed as a true bearing toward which you moved, and that drift is distance of current set divided by elapsed time
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