NavList:

Jeffrey Rock reffered to this story on 'PPrune':

https://www.pprune.org/military-aviation/649671-mag-grid-true-north-align.html 

This story from the UK Ordnance Survey is doing the rounds in the UK.  Last week it was picked up by The Times, The Telegraph, PPrune, and the RIN.  Good in a way that that it promotes interest in navigation, but poor in that it talks about ‘the three norths’ and completely ignores the ‘fourth north’, ‘compass north’, showing how little experience science correspondents have of practical navigation.   A magnetic compass does not align itself with magnetic north as per that published and checked by the Earth scientists but with the magnetic field in which it lies.  In short, the direction of the Earth’s magnetic field is altered locally around the compass by the presence of other magnets close by and the magnetic fields generated by nearby electrical equipment (hard iron effect) and the fact that the Earth’s magnetic field finds it easier to run through close-by iron objects thus modifying its direction (soft iron effect).  The difference between ‘compass north’ and ‘magnetic north’ is termed ‘magnetic deviation’.  The path for converting from ‘compass north’ to ‘true north’ goes compass direction, deviation, magnetic direction, variation, true direction, or vice-versa.  This in turn leads to the initialism CDMVT which has since been converted to more easily remembered forms of varying publish-ability in an inclusive setting e.g. ‘Cadbury’s Dairy Milk, very tasty’.  

Mariners and aviators have long gone to great trouble to ensure their magnetic compasses are sited in areas of minimum magnetic deviation and to tweak their compasses to remove as much of the remainder as possible.  A Vulcan Nav-Team was expected to be able to ‘swing’ an aircraft and reduce residual deviation of the aircrafts two main compass systems to less than 0.5 degrees, (a wonderful way to pass a spring morning but not so good in winter).  One wondered how reliable the result might be when the aircraft was swung with four Olympus engines shut down and only the Rover auxiliary power unit running whist being towed in a circle by a large iron towing tractor using an even larger iron towing arm.  However, hourly astro heading checks against the Sun or a star whilst airborne were always well within one degree, so we can’t have been doing that badly. DaveP