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Subject: Re: Mercator vs. Great Circle Charts
From: Steven Tripp (tripp@XXX.XXX)
Date: Wed Aug 29 2001 - 22:03:40 EDT
I have before me DMA #56 Great Circle Sailing Chart of the North Pacific
Ocean (a gnomomic chart). 38th ed. Sept 17, 1973 $1.50 !!!
> 1. Can true direction be determined in the same manner for all locations on a
> Mercator chart? How about on a great circle chart?
True direction (true course?) cannot be determined directly on the GC chart.
The lines of lat and lon are all distorted. The chart describes an indirect
method (below) for determining the GC course in segments.
>
> 2. How is the compass rose used on each of these charts?
The GC chart doesn't have a normal compass rose. Points on the chart are
transferred to a course diagram and then that lines is transferred to the
compass. This give the course for a segment of the journey.
If you don't have a copy of his chart you should get one. It's quite fun to
look at.
It says at the bottom that when two points differ less than 20° in lon you
must use the supplementary method (table-based) which is printed across the
bottom of the chart.
Steve T.
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