A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Dec 29, 15:03 -0800
Frank you wrote: 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
I can’t believe you said that. You seem to have very little idea of what a professionally trained navigator is responsible for. It’s not just being the person on a moving vehicle who gives the pilot or helmsman a course to steer. There are many types of projections, and as I have said earlier, they are all compromises, because it’s not possible to achieve all of the features of the ideal chart of a spherical body on a flat piece of paper. Every projection has its advantages and disadvantages The role of the professional navigator is to select the most suitable chart for the job in hand, be it operating in equatorial, temperate, or polar regions, be it for ocean plotting, map reading, or plotting radio bearings. In staff appointments this might involve advising upon and ordering the best projection for a future operation or specifying, in conjunction with the cartographers, the projection to be used for a new range of charts. I think the idea that one should try to draw a complete Mercator chart is clearly a non-starter for anyone except a cartographer, but one might, in an emergency, wish to extend an existing chart north or south, and that was probably the original reason for keeping the approximation techniques in syllabuses. As a sideline, successful navigation in difficult situations was always deemed to depend upon, amongst other things, self confidence, of being ready for anything, (Ready, aye ready!), and techniques such as the approximation methods are also there to inculcate such confidence (Just as the USN appears to be trying to do with the reintroduction of celestial training for officers).
You ask “what is a Normal Mercator chart for?” Well you know that. It’s one of the best charts for plotting rhumb lines in equatorial and temperate regions, because they’re straight lines. Witness the guide-lines on the early charts; all you have to do is transfer them with a couple of set squares. Try measuring rhumb line tracks with a protractor on a Lambert’s conical toppo. The track from A to B isn’t the track from B to A – 180. You have to remember to place your protractor on the middle of the line from A to B. On the other hand, plotting radio bearings from distant stations can more problematical on a Mercator chart, because they travel in great circles. So is searching for the source of distant radio transmissions, because then you’re considering lines of constant bearing.
Professional navigators do have an idea why they’ve chosen the chart they’ve chosen or been issued with, and even retired ones can, when they sit down to think about it, justify that choice. DaveP