As expert map readers will know, when you’re out and about navigating with a compass, there is a difference between magnetic north (where the compass points) and grid north (the vertical blue grid lines shown on OS maps).

The difference between magnetic north and grid north is often referred to as ‘grid magnetic angle’ and it not only varies from place to place, but changes with time. This needs to be taken into account when navigating with a map and compass.

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 east. The change started in the very south-west corner of Britain and will slowly progress across the whole country over the next 12 to 13 years. We wrote a blog on the progress of alignment between magnetic and grid north in Britain back in 2019, which you can read here.

Now, there is a third line 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 the 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 National Grid this central meridian line is 2°W or 400000m E.

The changing direction of magnetic north is now approaching this ’magic line‘ so all three will briefly be in alignment. Due to the magnetic north direction being skewed relative to the central meridian and the fact that it is moving, the triple alignment point will move up the central meridian.


The 'magic line' as it passes through Great Britain



https://www.ordnancesurvey.co.uk/newsroom/blog/magnetic-true-grid-north-align-over-great-britain?fbclid=IwAR35Zekr-NXZQTg1-E_VUkZ1adtTErUn_CtVllJfS8IG6PnhKNGR2fvNN_o