Whenever I give a talk or demonstration of map and compass work, the one element that always causes most confusion is that of North! We should all know that the top of the map is North – but which one?
There are in fact three different north’s! True North, Magnetic North and Grid North.
True North is the geographical north where the ‘north pole’ is, Grid North is the north to which the grid lines on your map points i.e. the top of the map, and Magnetic North is the north to which your compass points. As of 2017 this lies near Baffin Island NE Canada and is moving at approximately 30km a year.
Where the main confusion comes from is the naming of the angles between True and Magnetic North and Grid and Magnetic North. More specifically over the definition of the term Magnetic Variation (Declination). According to the book Mountaincraft & Leadership, the official manual for Mountain Leader Training England & Scotland and several other reputable sources, Magnetic Variation is the horizontal angle between Magnetic North and Grid North.
However, according to the Ordnance Survey and the British Geological Association Magnetic Variation is the angle between Magnetic North and True North. The angle between Magnetic North and Grid North is the Grid Magnetic Angle (GMA) and it is this angle that needs to be applied when converting between Magnetic and Grid bearings.
In the legend of the OS Landranger and Explorer maps gives details of the variation between the Magnetic and Grid north’s along with the date and rate of change in degree minutes per year. For example on my Landranger #185 it says the GMA is 2° 27′ west of grid north for 2011. Annual change is about 9′ east. There in 2017 the GMA has reduced by 54′ or virtually 1°. Therefore the current GMA for this map is about 1° 30′ (1·5 degrees) west.
How does that translate to the map and compass. Remember the phrase “From grid to mag add & from mag to grid get rid“. So you have taken a bearing on the map (grid) and want to use it on a compass (mag) we add the GMA (1.5 degrees)
Map Extract courtesy of the Ordnance Survey
Hill-Walking For The Over 60's 