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    Re: Newbie - Variation Question
    From: Trevor Kenchington
    Date: 2002 Feb 16, 16:42 -0400

    Got back from a business trip to the torrent of e-mails on variation,
    amongst which Andrew Denman had written:
    > so regardless of what convention is used (ie Easterly variation + or -) when
    > Easterly variation is applied to true, the resultant magnetic heading will
    > be less than true.  Is this the case?
    Since that seems to have been lost amidst all the rest:
    It isn't a matter of conventions, save for the convention of measuring
    headings and bearings clockwise from some reference meridian.
    Though hard to prove in an e-mail format, if you try sketching the
    various possibilities on a sheet of paper, it is easy to see that if
    magnetic north lies to the east of true north, the magnetic bearing MUST
    be numerically smaller than the true one (unless it is smaller than the
    true bearing plus 360 degrees, the latter being added to avoid negative
    angles). Likewise, if a particular compass' north lies to the west of
    local magnetic north, then a vessel's heading by that compass will be
    numerically larger than its magnetic heading.
    Re-label the variation and deviation how you wish, those relationships
    must remain correct.
    What would change them would be a reversion to the older way of
    expressing angles in terms of quadrants. If the true course were not
    "105T" but "South 75 East", the variation 16 degrees east and the
    deviation 2 degrees west, how many among us could swiftly determine that
    the compass course is South 89 East without first converting everything
    to 360-degree notation? I could not. [And for those who can do such
    calculations in their heads, try a true course of east by south a
    quarter south, easterly variation of one and a half points and westerly
    variation of a quarter point -- without either converting to degrees or
    counting the quarter points around the rim of a compass card!]
    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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