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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
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