How to take a bearing

A reminder on how to take a bearing – by Rob Johnson

how-touseCompass
How to use a compass

Grid North, Magnetic North & True North
There are three norths commonly in use in Great Britain:

Grid North: the direction of a grid line on the National Grid.

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

Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate.

Differences
The difference between True North and Magnetic North is called MAGNETIC VARIATION or DECLINATION.

Differences between the three norths are given on maps but check when they were printed as they can soon be out of date.

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, see http://www.geomag.bgs.ac.uk/gifs/gma_calc.html.