A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Jan 14, 18:11 -0800
GL wrote:I'm confused. The chart you posted is too small for me to read. You say that the diagram is for determining the "great circle bearing of the destination from the point of departure" but that is easily determined by measuring the initial great circle course at the nearest meridian printed on the chart to the departure. If it is determining the rhumb line bearing then it is apparently a graphical way of determining the "conversion angle," table 1 in Bowditch.
I'm not sure that it's as simple as that. The great circle chart is a planning chart for finding the en-route lat & longs of the great circle between A & B. No flat chart can represent a spherical chart perfectly. You always have to give up at least one property of ‘the perfect chart’ to get the property you want most from the chart. The great circle chart is a gnomonic projection. Great circles come out as straight lines, but at the expense of losing conformality. In other words the meridians and parallels frequently don’t cross at right angles; shapes aren’t always preserved; scale varies; and it’s not equal area. This makes measuring angles with a Douglas protractor and distances against a scale somewhere on the chart rather difficult, which is why it’s normally only used as a planning chart. The simple navigator can take the latitude where the straight line from A to B crosses each 10 degree meridian on the gnomonic chart and transfer those points to a Mercator chart, which is conformal. Then all you have to do is use your protractor and dividers on the ‘dogleg' sections of track in the normal way, and hopefully you won’t be far out from the true values. The ‘brainbox’ navigator can use the clever diagrams to do it all on the gnomonic chart. The actual curves must be a way of effectively transferring the track angle at each meridian to what it would be at the tangent of contact, where angles are preserved, but it’s a bit beyond me at I’m afraid. DaveP