Magnetic north has moved from west of Grid North to the east for the first time in 200 years, causing a massive reworking of the Ordnance Survey's maps.
While it's usually thought to be an eternal 'constant', Earth's poles are constantly shifting. Magnetic north actually points in different directions in different places due to variances in magnetic force, and map makers (or rather, map users) have to consistently adjust their work to account for the difference.
But there are some constants - even putting aside 'True North' (which is the direction to the geographic North Pole). For while its position has changed, Magnetic North has been west of Grid North by a few degrees for more than two centuries. In fact, it's been true ever since the Ordnance Survey was founded in 1791.
But now Ordnance Survey has announced that these poles? They are a changin'.
"The change is slowly crossing the country, but for now can only be appreciated in our Custom Made maps with a centre to the West of Penzance," it said in a blog post.
Gradually the change will move across the UK and be reflected on most maps - unless it changes back.
On its blog the Survey reminded users how to use a compass with a map.
"Place your compass on the map. Make sure the ‘direction of travel arrow’ is pointing in the direction of your route across the map. The easiest way to line the arrow is to place the side of the base plate so it crosses your starting point and the next destination of your journey.Suggest a correction
Carefully holding the compass base plate still, you will need to turn the compass housing so the orientating lines match up with the eastings (the vertical, north–south lines) on your map. Holding the map flat and the compass still, you need to rotate your body so that the compass needle settles in line (opposite) with the index line. To fully orientate your map you will need to make some adjustments for magnetic variation.
You can properly orientate the map by carefully turning the compass housing 4º clockwise (for example, depending on where you are in Great Britain and by checking on your map legend) and then turning your body again to realign the magnetic needle with the index line. Your map is now oriented to the north."