At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 7.8 knots. At 1035 your position is LAT 37°00.5'N, LONG 75°43.8'W. What was the set and drift?
• How to compute DR (dead reckoning) position from course and speed over a known time interval • Difference between DR position and observed (fix) position gives the current vector, whose direction is set and whose speed is drift • Using latitude and longitude differences to find north-south and east-west components of motion
• First, compute where you should be at 1035 if there were no current, starting from 0919, using the given course and speed. Then compare that DR position to the actual 1035 fix. • Convert the time run into hours, then use Speed = Distance / Time to find the DR distance run. Plot or sketch the DR track westward from the 0919 fix. • From the DR position to the actual 1035 fix, determine the direction of that line (this is the set) and the length of that line divided by the time (this is the drift).
• Make sure the elapsed time between 0919 and 1035 is correctly converted into hours and decimal hours. • Check the sign (east/west and north/south) of the latitude and longitude changes; remember increasing west longitude means movement to the west. • After you find the current vector, see which multiple-choice option matches both the direction quadrant (roughly NE, SE, SW, NW) and the approximate magnitude in knots.
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