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    Re: Old charts and traditional navigation methods
    From: Lu Abel
    Date: 2014 Aug 13, 09:14 -0700
    I think both this answer and the ones preceding it noting that compasses were divided into 32 "points" state something that is obvious from looking at the chart but don't really explain "why?" which was the focus of the original question.

    Without being an expert in technological instruments, I rather suspect that something as simple as crude protractors and/or parallel rules existed in the 16th century.   So why all these "bursts" of rhumb/compass direction lines when it would have been fairly simple to measure a course line with a protractor as navigators do today, or walk a line to a compass rose - again as navigators do today?


    From: Don Seltzer <NoReply_Seltzer@fer3.com>
    To: luabel@ymail.com
    Sent: Wednesday, August 13, 2014 5:11 AM
    Subject: [NavList] Re: Old charts and traditional navigation methods

    On 8/12/14, Jackson McDonald  wrote:
    > I have attached three photos of a French nautical chart dated 1771.
    > Like many old charts, this one contains numerous diagonal lines in addition
    > to lines of longitude and latitude.
    > How were these diagonal lines used?  How did they help a navigator plot his
    > ship's course?
    The diagonal lines are rhumb lines, and were used on charts with
    Mercator projections.
    They allowed navigators to plot courses of constant compass bearing,
    rather than the more difficult great circle route between two points.
    Don Seltzer

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