Three norths

Grid north is the direction of the grid lines on your OS map, up the page. True north points to the North Pole. Magnetic north is where your compass needle points. They are all slightly different, and the gaps change from place to place and year to year.

Map to compass: subtract (for now)

The angle between grid north and magnetic north is called the grid magnetic angle. For centuries magnetic north was west of grid north in Britain, so walkers added it. Since 2014 magnetic north has moved east of grid north, starting in the south-west. Where it is east, you subtract the angle when turning a map bearing into a compass bearing, and add it when going from compass to map. Today it is east across Wales, south-west England, the Peak District, the Lake District and most of Scotland. In much of eastern England it is still slightly west, so there you add. The British Geological Survey's rhyme for this is "East is least, west is best". Every degree you get wrong puts you about 17 metres off line for each kilometre you walk, so it is worth getting right. This tool tells you the angle and which way to apply it at your exact spot.

Grid references

Britain is split into 100 km squares, each with two letters, such as SH for Snowdonia. The numbers give the distance east (along the corridor) and then north (up the stairs) inside that square. A 4-figure reference is a 1 km square, 6 figures is 100 m, 8 figures is 10 m and 10 figures is 1 m. A grid reference names a square, so when you type a short one we use the middle of that square.

How accurate is it?

Your phone gives a GPS position. We change it to the OS National Grid using Ordnance Survey's published Helmert transformation, which OS says is accurate to within 3.5 metres (95% of the time). That is usually smaller than the error in the phone's own fix. Magnetic north comes from the World Magnetic Model 2025 (WMM2025), made by NOAA and the British Geological Survey. It is a global model, good to a fraction of a degree. Distances are straight lines over the ground, as the crow flies, and do not include climbing. Grid bearings are measured from grid north, just as you would with a protractor or compass on the map.