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    Re: Accuracy/precision in plotting tools.
    From: Fred Hebard
    Date: 2004 Mar 30, 10:35 -0500

    I noticed that the position plotting sheets from Celestaire are not
    precise.  Is this the error you are finding?  You could check your
    triangle on a government chart, which one would assume is accurate.
    You also can construct a right angle with a compass and straight edge:
    draw an equilateral triangle, then bisect the base and run up to the
    apex; it might be easier to start with the compass half open, to
    bisect, then spread to get full width.  That would give you a check on
    the triangle, which I would assume is correct as opposed to the chart.
    If you did it on the chart, you could check the rose.
    On Mar 30, 2004, at 10:03 AM, Jim Thompson wrote:
    > Preamble:
    > 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
    > jim2@jimthompson.net
    > www.jimthompson.net
    > Outgoing mail scanned by Norton Antivirus
    > -----------------------------------------

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