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So now that I know why I use statute miles, I am curious as to why all my charts are Polyconic charts and not Mercator projection charts. If they were, I would probably use nautical miles. For this answer, I have to dig back in history.
From the book “A Textbook onCoast and Lake Navigation” Copyright 1902 by International Textbook Company. Can be downloaded free of charge as a PDF from “Google Books”
From page 38:
The polyconic “projection is therefore advantageous for the representation of a coast line that runs north and south, or in the direction of the meridian, and is for this reason extensively used by the United States Coast and Geodetic Survey in the preparation of working charts of the coast, and also by the United States Engineer’s Office, War Department, in the preparation of charts used by mariners on the Great Lakes.”
From page 43:
“Charts for Use on the Great Lakes….. Great Lakes charts published by the US Engineer Office are constructed on the polyconic projection… on the US Engineer chart only statute mile is used.”
From these two statements we know now why Polyconic charts were used and why the scale has traditionally only had Statute mile as a unit of measure. Polyconic charts were used because of predominant north-south coastlines of Lakes Huron and Michigan. The scale of statute miles was just decreed. I read probably a dozen old navigation books and found this same justification in a number of them.
Now, two more questions came to mind. First, why is Polyconic useful for north-south oriented coast lines? Secondly, are statute miles associated with Polyconic charts?
From “United States Hydrographic Office” “Great Lakes Pilot, Volume II” 1921 Government printing office. Can be downloaded free of charge from Google Books.
From Page 3:
“On a polyconic chart, since a straight line represents (within the limits of 15 or 20 degrees of longitude) nearly the arc of a great circle, or the shortest distance between two points, bearings on the chart are identical with observed bearings. All Lake Survey charts are polyconic projections. The Mercator projection is unsuited for surveying purposes.”
This is an incredibly powerful statement and answers the reason for polyconic charts used in north-south coastlines. The reasoning is that navigating along a coast invariably meant shooting bearings to objects on the coast to determine position based on triangulation.
Great circles define the bearing to an observed object. Great circles are straight lines on polyconic charts. Great circles are curved lines on Mercator charts. If we want the most accurate triangulation, use a polyconic chart which explains why surveyors cannot use a Mercator chart. The error in triangulating on a Mercator chart is greatest in the east-west direction; that is facing a north-south coast.
And following visual aids causes us to follow a great circle, so plotting the course on a polyconic chart shows the obstacles that might be crossed during the transit.
Last edited by BryceGTX; 09-20-2011 at 05:50 AM.