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    Re: Bowditch Table 15
    From: Bill B
    Date: 2005 Jan 24, 16:57 -0500

    Jim wrote
    > To further confuse matters, see the difference in definition for the
    > entering value height of object in the NGIA online calculator at
    >  I emailed NGIA to
    > obtain their underlying formula for that calculator.
    If I understand, we are discussing the table (15 in my copy) that determines
    distance from an object by using the vertical angle between its top and the
    horizon.  In this case the base is below the horizon (or, I can see the mast
    top but not the hull of a sailboat).
    First, the sextant reading is adjusted for index error and dip, but nothing
    else. Imagine the dip correction is what may yield negative values for the
    As a non math-major I'll do my best to represent the formula from Bowditch
    in ASCII:
    D = (square root of (tan angle/0.0002419)squared)) + H-h/0.7349 - tan
    The constants 0.0002419 and 0.7349 account for standard terrestrial
    H is height of object above sea level in feet
    h is height of observer's eye above sea level in feet
    If I read the explanation correctly, the distance (D) is from the observer
    to the object, not distance from object to observer's horizon.  (Perhaps the
    dip correction helps out here as well?)
    At any rate, will let those with appropriate skill sets chew on the
    Hope this helps.

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