The three main types of north are:
• Magnetic North – is north as defined by a compass or the magnetic pole of the world. Magnetic north is currently somewhere in the Hudson Bay in northern Canada and is moving northwest at about 40km per year. However, do not expect it to end up in Australia. Over the millennia it has reversed direction, sped up and slowed down.
• True North – north as defined by lines leading to the poles, or to the ‘top’ and ‘bottom’ of the world. All lines of longitude point to true north.
• Grid North – north as defined by your map’s gridlines, usually UTM, but in other countries like the UK, perhaps OSGB (British Grid system). As above, longitude lines are the same as true north. Only one UTM northern line in a UTM gridzone aligns with true north and that is the false easting in the centre of the gridzone. The rest of the lines are up to three degrees out.
To make it interesting the three are unlikely to be the same. There may also be a ‘User Defined North’ in case you do not agree with the receiver’s definition of magnetic north and you want your own.
The angles between the norths have names:
• The difference between magnetic north and true north is called ‘magnetic variation’ or ‘declination’. Note that sometimes magnetic variation is the difference between magnetic north and grid north. A better term is ‘grid/magnetic angle’ as at least then you are sure you know which two norths you are referring to.
• The angle between true north and grid north is the grid convergence.
Note: ‘declination’ can also refer to the latitude of the stars and the vertical angle between the horizontal and the earth’s magnetic field.
Why do you need to know?
Because if you take a bearing on a ‘north’ and it’s not the one you think it is, then your bearing will be out and you won’t end up where you want to end up.
Tip: the photo shows a compass being used outside of the vehicle. That’s important so that the vehicle’s electromagnetic fields do not interfere with the compass. And, the compass will show magnetic north!