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Grid North, Magnetic North and True North

There are three Norths commonly in use in Great Britain.

Grid North the direction of a grid line which is parallel to the central meridian on the National Grid.

True North is the direction of a meridian of longitude which converges on the North Pole.

Magnetic North is the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.

Differences

The horizontal angular difference between True North and Magnetic North is called MAGNETIC VARIATION or DECLINATION. The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC ANGLE. It is this angle which needs to be applied when converting between magnetic and grid bearings.

Differences between the three norths are given on our 1:50 000 scale OS Landranger and OS Explorer 1:25 000 maps.

On OS Landranger Maps, the difference between True North and Grid North is given for each corner of the map and is shown in the legend as is the difference between True North and Magnetic North. The information is shown diagrammatically in the middle of the North margin. In both cases the year for which the value was compiled is given and once its value has been corrected for the current year, its value can be accepted for the whole sheet.

Calculating magnetic variation

Example – Penzance

The Magnetic North to Grid North is predicted to be 0° 00' west of grid north at the centre of the sheet in July 2014, with an estimated rate of change of 11' east a year.

The difference between True North and Grid North at the sheet corners are given and, by interpolation, the difference at Penzance (approximately central to the map), is that True North is 2° 40' east of Grid North.

Thus the predicted difference in 2014 between Magnetic North and True North is 2° 40' – 0° 00' plus 2° 40'.

Example – Lowestoft

Magnetic North to Grid North is predicted to be 3° 18' west of grid north at the centre of the sheet in July 2014, with an estimated rate of change of 10' east a year.

The difference between True North and Grid North at the sheet corners are given and, by interpolation, the difference at Lowestoft, which is close to the SE corner, is that True North is 3° 00' west of Grid North.

Thus the difference between Magnetic North and True North is predicted to be 0° 18' in 2014 – 3° 18' minus 3° 00'.

Grid magnetic angle calculator

The magnetic data shown on Ordnance Survey maps are supplied by the British Geological Survey (BGS). The BGS website includes a tool which will calculate the angle between grid north (British National Grid) and magnetic north at a given location from either coordinates or post code

For more information see the BGS Magnetic angle calculator.

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