A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Dec 29, 11:50 -0800
David C, You wrote:
Thanks for being persistant (I think I have seen this subject discussed several times in the archives). I think I finally understand it.
- A conformal map preserves angles locally.
- A mercator projection has a variable lat/long scale based on meridional parts. It is locally conformal.
- Projections other than mercator may be locally conformal.
If I draw a plotting chart with a long = cos(fixed lat) scale I have drawn a locally conformal (but not Mercator) chart. If I open Norie at the Meridional Parts table and create a chart with a long proportional to (difference meridional parts) scale I have drawn a Mercator chart. This chart is conformal.
Yes, that's all right on the money! Standard small plotting charts, sometimes known as "Universal Plotting Sheets" are conformal but not Mercator. Also the sections in Bowditch and other resources that describe how to construct Mercator charts are not providing instructions for constructing simple plotting charts. It's not clear if the authors understood that, but it's certainly the case that many, many readers have misunderstood. It's interesting that the obsession with Mercator charts in navigation has become almost ritualistic. Navigators are so sure that they must have a Mercator chart that they feel compelled to learn and follow those instructions for drawing one's own Mercator chart. But they have not idea why they are doing this. What is a Mercator chart for? Why would you ever need to construct your own? Most navigators never get around to answering that question properly.
Conanicut Island USA