Three Norths: Magnetic north, true north and grid north are lining up in Britain

摘要

In 2014, for the first time in Great Britain since the 1660s, magnetic north moved from being to the west of grid north to the eastThe change started in the very south-west corner of Britain and will slowly progress across the whole country over the following years. Now, there is a third north about to come into alignment-true north.This is the direction of lines of longitude that all converge at the north pole. On a map projection like the transverse Mercator projection used for British National Grid, the longitude lines curve away from the straight grid lines.This is called "convergence".The amount of curvature varies across the grid area, except at one line. A transverse Mercator projection has a"central meridian" line where a chosen longitude aligns with a vertical eastings grid line. For the British National Grid this central meridian line is 2°W or 400000m E. The changing direction of magnetic north is now approaching this central meridian so all three norths will briefly be in alignment. Due to the magnetic north direction being skewed relative to the central meridian and the fact that it is changing in time, the triple alignment point will move up the central meridian.