A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2016 Dec 26, 11:20 -0800
I understand that for absolute accuracy the small scale plotting sheet should not be called "Mercator" but then why, in all these years, has there not been a different name for it put forward in any of the navigation texts? You propose several names that I have never seen before in any text. Bowditch says that you can make a Mercator small area plotting sheet by using Meridional parts and then goes on to say "a good approximation can be more quickly constructed" using the described methods. "A good approximation" of what, of a Mercator chart. I think you may be being overly pedantic on this one, Frank.
Time to scream. Arrrrrgh. No, that is exactly the big, fat error that navigators routinely make. Local scaling of longitude minutes to match latitude minutes is not the Mercator projection. That property describes a map that is locally conformal, but various map projections are locally conformal, not just Mercator. If you don't like the name "conformal," which is unfortunately not very transparent in meaning, then call it "longitude-scaled." Another name for it would be "similarity". On a locally conformal map projection squares look like squares, and triangles look like triangles. That is, the shapes portrayed on the map are "similar," in the sense of "similar triangles," to shapes drawn on the ground, in the real world. Note that this only applies locally, in a small area, on a Mercator chart. The best example of a failure to preserve shapes in the global Mercator chart is the rhumbline itself which is a spiral, a loxodrome, on the globe, but a perfectly straight line on the chart. In each very small local region shapes are portrayed correctly, but globally they cannot be.