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 » Online Classroom   » Inland and Coastal Navigation   » Public Discussion of Inland and Coastal Navigation   » 5-7

Author Topic: 5-7
 M. C. Rowley posted August 14, 2005 06:18 PM                   Since we know two sides of the triangle and the angle between them why not simply use the Law of Cosines to calculate the third side (our distance error) instead of plotting? A 12 degree error at 45 miles would place us off course 9.4 miles.Mark RowleySanta Fe, New Mexico
 David Burch posted August 16, 2005 02:03 PM                   the goal here is not necessarily use a formula that requires computation, but something that you can work in your head for a quick answer. And that is where the 6° rule comes into play. It states simply that a 6° right triangle has sides in proportion of 1 to 10. (ie a way to memorize the tangent of 6°). This then can be scaled down forever and up to about 18°, where the ratio is not 1 to 10 but 3 to 10. A 3° angle has sides in proportion of 0.5 to 10, and so on.Twelve degrees is 2 to 10, or 0.2 x 45 miles = 9.0 as a perfectly good approximation to the rigorously correct answer as we will never know the precise values that go into such a long run. From: Starpath, Seattle, WA

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