A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Jan 15, 02:20 -0800
For practical navigation just place your plotter's straight edge on the straight line of the GC on the gnomic chart and read out the course against the closest meridian to the departure. Then slide the plotter along and read the courses at other meridians. Just like reading the course on standard flight charts. gl
You can’t always do that on a gnomonic chart Gary. The example doesn’t look too bad, because the eye is taken to the centre of the chart, which is also the tangent of contact, but the lack of orthagonality gets worse the closer you get to the edges of the chart. Look in the SW corner. Where would you sit your Douglas protractor? In the sectors 360-090 and 180-270, ninety degrees indicated on the chart is clearly less than ninety degrees on a protractor, and in the sectors 090-180 and 270-360 is more than ninety degrees measured with a protractor. I.e. the chart isn’t conformal, and the normal techniques of chartwork can’t always be used.
Just to add to my earlier comments, which were written in haste well after my normal bedtime. Gnomonic charts aren’t just used to plan navigation trips, although Lindbergh used one to plan his Atlantic flight very effectively. They can be used for any activity which involves great circles. E.g. pointing directional aerials or pinpointing seismic activity. For navigational use, the lack of orthogonality means that angles aren’t preserved, shape isn’t preserved, and scale alters at different rates in different directions. By way of a contrast, the Mercator chart is conformal. Although scale changes, it does so at the same rate in each direction so shape is preserved at the expense of the chart not being equal area. E,g. On some Mercators, Greenland appears nearly as big as Africa. DaveP