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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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