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    Accuracy/precision in plotting tools.
    From: Jim Thompson
    Date: 2004 Mar 30, 11:03 -0400

    I bought a new Portland triangle and new parallel rules in an effort to draw
    inordinately accurate course lines for an upcoming exam.  But when I measure
    the compass rose printed on an exercise Mercator plotting chart, either the
    large compass rose in the center of the chart is skewed, or the triangle is
    off, or a little of both.
    When I measure the rose on the 000-180 axis with the triangle, it gives
    those angles precisely.  But when I measure the rose on its 270-090 axis,
    the triangle gives angles off by 0.5 degrees or more.
    This means that the rhumb line I draw by walking my parallel rules from the
    compass rose to the desired course line has an angle significantly different
    from a course line that I lay off with the Portland triangle: more than half
    a degree different, or 1 mile over 10 inches.  Which is right?
    I measured that angle with 4 other smaller, cheaper protractors that I have
    around the house.  They all seem to report an angle greater than 90 degrees,
    which would suggest that the rose is printed in a skewed fashion on that
    chart. But when I apply the Portland triangle to compass roses on two other
    small scale Mercator charts (North Atlantic and Caribbean), I obtain similar
    findings.  These results seem irreconcilable, unless all four protractors
    are built wrongly with the same error, or the compass roses on a Mercator
    chart are not perfect circles, or ...?
    My questions:
    1. What are the accuracy and precision that I can expect from a charting
    tool like a reasonably well made Portland triangle from a reputable company?
    2. Are compass roses on small scale Mercator charts not perfect circles?
    Jim Thompson
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