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  • #16
    Originally posted by Wldrns View Post
    Technically it is a trigonometric problem, But for an easy rule of thumb(for relatively small angle errors), you will have approximately a 1 in 60 cross course distance displacement error for each degree of heading error. In other words, consider traveling 1 nautical mile (about 6,000 feet). one part in 60 of 6000 is 100, therefore you will be displaced from your goal by 100x13=1300 feet missing your target. in your case ( in this region of the country) you will be that much too far to the left of target if you have ignored magnetic declination.
    Using graph paper and a protractor I came up with 387 yds, albeit I was using a 5280 ft. mile. Not to mention a ragged protractor my son used probably as a Frisbee whilst in grade school 20 years ago.
    Still 1300 ft. in the forest is quite the distance. I am sure that simple is the best way to start someone with a compass as long as they realize the limitations.
    Thanks for your response and help, much obliged.
    "A culture is no better than its woods." W.H. Auden

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    • #17
      Originally posted by geogymn View Post
      Using graph paper and a protractor I came up with 387 yds, albeit I was using a 5280 ft. mile. Not to mention a ragged protractor my son used probably as a Frisbee whilst in grade school 20 years ago.
      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.
      Last edited by Wldrns; 10-05-2016, 08:25 AM.
      "Now I see the secret of making the best person, it is to grow in the open air and to eat and sleep with the earth." -Walt Whitman

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      • #18
        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!
        "A culture is no better than its woods." W.H. Auden

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