A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Sep 20, 01:32 -0700
I think, as with engineering solutions, in a great big World, over several centuries, with various applications, paths will intertwine. Air navigation varies considerably from maritime navigation. In maritime navigation tidal flow can be predicted years in advance; hence tidal flow tables can be produced. Also the distance travelled by an aircraft in one hour might be 100 times further than that which might be travelled on the sea.
I wasn’t aware that at sea ones DR position frequently ignores the effect of tidal flow. There is some sense in this in that over a period of time the effects of tidal flow cancel out, notably crossing the North Sea in a yacht. It’s not always so, otherwise there’d be no point in castaways launching messages in bottles, and tidal spurs such as Spurn Point wouldn’t form.
In air navigation the equivalent of tidal flow is wind, which varies greatly with time and place and is rarely predictable for more than a few hours in advance. Therefore, in air navigation ones DR position tends to include the effect of wind. In the ‘manual air-plot’ which some attribute to Francis Chichester, one marks off ones heading and airspeed x time flown to give an ‘air position’ then adds a vector representing your forecast or recently found wind to give your ‘DR position'.
Aviators have their own terms too. If you have to divert and you’re pretty certain where you are at that moment you make a ‘snap alteration of heading' immediately. If you’re less certain exactly where you are or if you want to make a really good job of calculating your new heading, that’s going to take a little more time, so you keep going in the original direction but ‘DR ahead’ to calculate exactly where you’ll be at some point in the future, six minutes was commonly used in the days when aircraft could barely manage 180 kts.
But confusion has always been with us. I can remember as a young child singing hymns at Sunday School wondering why on Earth anyone would want to put a city wall around a green hill far away in the first place, or my disappointment after breaking into a bag of my mothers self-raising flour to find it didn’t contain the longed for raisins after all. DaveP