A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2019 Jan 21, 16:41 -0800
RE : Flying-Antarctica-Morris-jan-2019-g44044
" I am not quite sure what they mean by "true direction". Is that the instantaneous course on the standard latitude/longitude globe? I think so. ": Yes, True direction is a direction referenced to the geographical North.
" My reference states that the grid system simplifies in that the wind direction remains constant. " : This statement needs clarification. As long as the high altitude winds follow a great circle track - which may often be [approximately] the case on rather short distances (e.g. up to 1,000 NM), but not always since jetstreams sometimes sharply "turn" generating [strong] Clear Air Turbulence - then the "grid wind direction" remains constant since on the Aircharts used for transpolar navigation any Great Circle is very close from a straight line. Since straight lines cross the airgrid at the same angles, then - yes - the grid wind direction remains [approximately] constant.