The Ordnance Survey recently announced that, in the south-west of England, magnetic north now lies to the east of grid north (see: tgomagazine.co.uk/viewpoint/magnetic-north-to-move-on-os-maps). In case that statement has left you scratching your head in bafflement or blushing at the fact that your nav skills need revising, here’s a quick reminder...
The needle on a compass is attracted by magnetic force, which varies around the world and changes constantly. The difference between ‘grid north’ (north as indicated on maps) and magnetic north needs to be taken into account for accurate navigation, particularly over long distances. (Where the difference between the two is small and you’re only walking a short distance, you may choose not to compensate for it).
For many years, magnetic north has been a few degrees west of grid north in Britain, but the difference has been decreasing. It also varies throughout the country, which is why every Ordnance Survey Landranger or Explorer map includes information on the grid magnetic angle for the relevant area at the time of publication (the maps also show ‘true north’ – the direction of a meridian of longitude which converges on the North Pole, but you can ignore that for UK navigation purposes).
Walking on an accurate bearing
1. After choosing a route on the map, place your compass on the map with the direction of travel arrow pointing in the direction you plan to walk. Use the edge of the compass or the parallel lines inscribed on the base plate to line up the direction as precisely as possible.
2. Holding the map against the compass and keeping the map flat, turn the compass housing while keeping the base plate still, so that the orienting lines match the eastings (vertical grid lines) on the map and the orienting arrow points to north on the map.
3. Compensate for magnetic variation – if magnetic north is west of grid north, you need to add the relevant number of degrees to the bearing you have just taken (indicated on the dial of the compass housing, in line with the direction of travel arrow). If it is east of grid north (currently only the case in the far south-west of England) you need to subtract.
4. Hold the compass with the direction of travel arrow pointing straight ahead from your body. Turn yourself around until the compass needle matches up with the orienting arrow.
5. Walk in the direction indicated by the direction of travel arrow.
There’s a useful tool on the British Geological Survey site to help you calculate the variance for any location: http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html