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    Bowditch Table 15
    From: Trevor Kenchington
    Date: 2005 Jan 23, 00:36 -0400

    I am gradually working my way through Bowditch (1995 edition), trying to
    be sure that I understand everything that is relevant to smaller vessels
    that the book is really intended for. Most I can follow, sometimes with
    a bit of effort, but I am having trouble with Table 15. Since the table
    numbers have changed between some editions, Table 15 in 1995 was the one
    providing "Distance by Vertical Angle Measured Between Sea Horizon and
    Top of Object Beyond Sea Horizon". (That was Table 9 in at least one
    edition of Bowditch.)
    The text description accompanying the table says that it provides the
    distance to the object (by implication: from the observer to the
    object), given a knowledge of the difference in height between the
    observer's eye and the object, plus a vertical angle measured between
    the top of the object and the horizon (that angle being corrected for IE
    and dip). [The Bowditch main text doesn't seem to refer to Table 15 and
    mixes this business of an object beyond the horizon with the common
    business of distance off by vertical angle of an object of known height
    whose waterline is visible.]
    Table 15 itself does give distances from observer #1 to the object
    observed, if the observer #1 has zero height of eye and the top of the
    object is just dipping below the horizon. At least, the tabulated
    distance then equals the horizon distance for an observer (#2) at the
    top of the object, which should be the same thing.
    However, if Table 15 was to be believed, as observer #1 climbs the mast
    of his boat and increases his height of eye, the object observed would
    drop below his horizon (assuming its true distance did not change),
    which is obviously nonsensical.
    So ... if Table 15 does not give the distance from observer to object,
    except in special cases, what does it provide?
    I'm guessing that it might be the distance from the object to the
    observer's horizon, to which the observer's horizon distance must be
    added to get the full distance from observer to object. However, I do
    not know of any way to either confirm of refute that idea (save for a
    full-scale experiment, which is a bit beyond my resources just now).
    Can anyone sort this out for me?
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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