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Old 10-05-2016, 05:08 PM   #18
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Join Date: Feb 2007
Posts: 1,861
Originally Posted by Wldrns View Post
Of course your compass is really just a protractor, so you could just as well use that on the graph paper(or a map), probably more accurately than using a frisbee. Note the approximation trick I gave you is fairly accurate only for truly small angles. the actual formula is S=r*theta, where S is the cross track error distance, R is the distance traveled, and theta is the error angle in radian measure. The real number is 57.3 degrees, not 60 as the number of degrees in a radian(mental math is easier when using 60 than with 57.3) The first term from the Taylor series small angle expansion for the trigonometric sine angle gives the shortcut. 13 degrees may be a bit large for the approximation to accurately hold.
(BTW - Using the acccurate form of the trig sine formula gives 399.3 yards as the answer over 1 statute mile of travel.)

Don't neglect magnetic declination. it is depicted on every topo map.
Yeah, That Trig stuff is beyond my comprehension. I am glad someone has it figured out. I guess I was trying to make a point that a simple compass has limitations that a newbie should be aware of. If a newbie is targeting a lake a mile away, make sure that lake is 2600' wide and aim for the middle if you choose not to use declination.

Again thanks for your help!
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