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