Oooo my 'ead 'urts!
I wish I hadn't started reading this thread, but I simply couldn't stop myself.
My late dad was a navigator, by all accounts a very good one and he would have loved this thread.
I have always been comfortable teaching my Scouts how Latitude and Longitude work and how a nautical mile is a mathematical subdivision of a circle the diameter of the Earth and so on. This all leads naturally enough to the various grid systems in use, down to the beloved Ordnance Survey grid, which I am trying to teach them to understand and use.
But now I've read this thread and I find I am going to have to lie to my Scouts, because there is no way I'm going to explain this!
Nevertheless, even though my brains are now mush and I'm not sure whether to trust my hand held GPS any more, thanks to everyone who contributed to a very interesting thread and the links to pages of mind boggling information.
Roger.