NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Mercator vs. Great Circle Charts
From: Nigel Gardner
Date: 2001 Aug 29, 3:15 PM
From: Nigel Gardner
Date: 2001 Aug 29, 3:15 PM
In spite of the rather unfortunate initial exchanges, an interesting point is raised about what is meant by 'great circle charts'. Probably the most commonly used is Lambert's conformal, although not strictly a GC chart because of distortion away from the standard parallels is near enough for practical purposes, gnomonic projections (although correct) suffer from scale exaggeration away from the centre; esoteric ones such as the Mecca azimuthal or the Rugby diagram are precise but only from or to the one point. Interestingly when I was scratching a living navigating aircraft, for crossings between UK and Canada we calculated the latidude intercept for every 5deg of long. plotted these on a mercator and flew each leg as a rhumb line, any HF radio bearings that we got were plotted using the conversion angle. Other oceanic crossings tended to be at lowish latitudes so a mercator was good enough. A friend of mine was a master on super-tankers and required the mate of the watch to take a sun sight each day from the deck watch, arguing that GPS might get switched off or the nav equipment might suffer a major failure. NG