If this is the first time you are joining the Map Reading series, you should start from the first post.
This time we are touching a slightly more complicated and technical subject: true north vs grid north vs magnetic north. The very fact that we have three different Norths is a hint to the problem in hand, but worry not, we will cover all three separately, how they are measured and how they relate to each other.
True North (Geographical North)
True north is generated by the earths rotational axis – as the planet rotates, it does so on an axis that can be imagined as running from the center of the North Pole, through the earth’s core to the center of the South Pole. The true north is actually the northern most point on Earth.
On the map, the true north runs on the longitude line that meets at the north pole. The best way to think about it is that in order to see the true north on a map we need to put the map on a ball, which will give a slight distortion of the grid lines, especially with high scales.
Grid north is the simplest one to understand – it is the straight up pointing north on the map. In almost all maps the north is pointing up from the direction of reading and it is aligned with the grid lines.
This will be slightly confusing, even more than the true north definition. The magnetic fields that generate the “pull” for the compass’ magnet is generated by solar winds and the earth’s rotation. The solar winds create a magnetic field in the northern cap of the planet, but magnetic north is not at the north pole. The magnetic force is slightly “off” relative to the geographical north and actually moves regularly. At the moment, magnetic north is around Ellesmere Island in Northern Canada.
Since the magnetic fields change yearly, it is important have an up-to-date map (more on this below).
The relationship between the Norths
Declination (Magnetic Variation)
Declination is angle that is created in various locations around the planet between magnetic north and true north. Each location has a unique declination which needs to be accounted for when map reading via a correction angle. The UK ranges between 2°-6° east while the USA currently ranges from 60° west to 45° east.
As the magnetic north moves, declination needs to be updated regularly and up to date in formation can be found for most countries. Some tools are:
NOAA calculator for the USA
BGS calculator for the UK
NRC calculator for Canada
All you need is to search in Google (or any other search engine) “declination data for …”
The declination will be shown in the map legend featuring a clear diagram of the correction angle and the actual degrees of correction. It is important to account for the declination before taking any bearings.
Grid magnetic angle
This is the angle between the grid on the map and the magnetic north. The grid and magnetic angles tend to be different from the declination and they will both be displayed in the legend in a schematic way.
Practical bottom line on correcting Norths
The big problem with this information is that at the end comes the simple question, so how do I get it right? What angle do I need to apply, and in which direction? We will use the example above.
1. The legend states that at the center of the sheet the true north is 0°51′ (0 degrees and 51 minutes). This means true north is 0.85° west (fun fact: each minute is 0.0166° west, so 51*0.0166=0.85°).
2. The legend also states that the magnetic north is 1°54′ west (1.9°) for 2011 and is estimated to move east 09′ every year, which means we have 1°18′ (1.3°) difference between true north and magnetic north.
3. To correct the readings, every time we use the compass in that area we need to account for a 1.3° change. If we are pointing west to the north-south line, we need to subtract 1.3° and east of the line we add 1.3°.
I know all this sounds very complicated and it took me a long time to understand how to account for magnetic changes, but we will go over it more in detail in future posts where we will be talking about compasses and how to use them.
I hope you have managed to understand the three different Norths and if you have a question relating to this, just leave a comment and I will try and clear things up a bit more.