Magnetic Declination and Compass Navigation

Which type of north does a compass needle point and what difference does it make?

This week with our Explorer Scouts, we did some work towards our Navigator Staged Activity Badge and part of this was a quiz which we made with various questions about compasses, maps, map symbols, and a little bit of route plotting. One of the questions we asked the Explorer Scouts was “Where does the red needle of a compass point” and the answer threw a lot of them off so we thought we would explain what north really is. Hopefully, this post proves useful for somebody, somewhere.

 

So where does the needle on a compass point?

The obvious answer is north, however, there are different kinds of north:

  • Grid north
  • True north
  • Magnetic north

Grid north is the direction that an Ordnance Survey map points. True north is the direction of the meridian which points to the north pole. A conventional magnetic compass that we use in land navigation points to magnetic north. It is attracted to the northern magnetic pole of the earth.

In England, magnetic declination or magnetic variation is small so it’s generally not a problem but that doesn’t mean we shouldn’t understand it to maximise our navigation skills and knowledge, right?

What is grid north?

Grid north, the direction of Ordnance Survey maps are closely linked to true north. Ordnance Survey maps always point grid north. Vertical grid lines on the map, those going from bottom to top, follow a northerly line through the central meridian of the UK which is based on a system called Ordnance Survey National Grid or British National Grid (BNG) as shown below. The central meridian for the UK is 2 degrees west.

The Ordnance Survey National Grid or British National Grid (BNG)

The red line on the map above is the central meridian: a line through the country which follows a notherly track to the north pole through the centre of the UK. In Basingstoke, we feature on the SU Ordnance Survey map which, as you will see, is on the edge of that central meridian which means we have it easy as you will see in the true north section.

What is true north?

True north is the direction of the closest meridian of longitude, a line that goes south to north, which joins up with the north pole. As you travel further west of the 0 degrees line or further east of the 0 degrees line, the meridians closest to you start to change shape and instead of being parallel lines, start to become somewhat trapezoid shaped: we know, it’s a bit confusing.

In the UK, for all real purposes, we can ignore the whole grid north vs. true north debate. If you wanted to be exact about it, however, an SU map of Basingstoke has the following statement printed on it: “At the centre of the east and west sheets true north is 0 degrees and 47 minutes, and 0 degrees 34 minutes west of grid north respectively. The annual change is approximately ten minutes east.”

In simple terms then, true north is less than half a degree west (negative) of true north on our map. Every year, that number changes by approximately ten minutes (1/6th of a degree positive). Eventually, then, those 47 minutes will become zero and as time goes even further, will become a positive variance.

What is magnetic north?

Magnetic north is the one we do need to think about in the UK because even though our variance is small, it can still make a difference. The earth is round, we know that. We also know that at the top is the north pole and the bottom is the south pole. Magnetism generated by the earth is a result of magma flow inside the earth’s core and various other things. The problem this creates, however, is that the top of the magnetic field isn’t exactly at the top where true north points. It’s slightly off-centre and it’s actually always moving very slightly. This difference is what we call the magnetic declination.

When we use our compass for navigation, we lay our compass on our map to get our bearing to walk to. We spin the bezel to line our start point and our destination point, rotate ourselves and the compass around that point and start walking. The problem then is that our compass points to magnetic north and our map points to grid north (let’s just say it’s true north for argument’s sake in the UK). If we were to follow that bearing over a very long distance we would eventually start to veer off-course.

The level of magnetic declination in the UK is calculated by the British Geological Survey (BGS) and they publish terribly complicated looking maps called Isogonic Charts which record the variance. Luckily for us, Ordnance Survey maps report the magnetic declination on them and we can use an online tool to get the very latest data too.

For example, the SU map for Basingstoke carries the following statement: “At the centre of the east and west sheets magnetic north is estimated at 2 degrees and one minute and one degree 54 minutes west of grid north respectively. The annual change is approximately ten minutes east”. The online tool at http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html, using SU 480560 as the grid reference, tells us that the BGS value, as of July 2018 is 1 degree and 14 minutes west (negative).

How to account for “north”

Now that we understand the different types of the north, how do we account for it and deal with it? It’s actually surprisingly simple.

Grid north we ignore as that is the north we are already working with when reading a map. True north has such a small angle in the UK it is not worth thinking about. Magnetic north, as we demonstrated above, for Basingstoke, is just over one-degree negative, to the west.

For a westerly magnetic declination, we need to add degrees to our compass. For easterly magnetic declinations, we need to subtract degrees from our compass. The diagram on OS maps helps us remember this too. In the illustration below, the map being used has easterly declination which means you need to subtract degrees.

If the magnetic north and true north arrows are pointing to the left of grid north, we need to add minutes: left is more, right is less. This works on your compass too! If you rotate your bezel left, you will be adding degrees. If you rotate right, you will be subtracting degrees: left is more, right is less.

Ordnance Survey map north indications for declination.

Here are a couple of examples, some of which have been borrowed from the Ordnance Survey at https://www.ordnancesurvey.co.uk/resources/maps-and-geographic-resources/finding-north.html.

If you are walking Snowdon, and you are at 53.0682389 degrees latitude and -4.0766629 longitude, the magnetic declination is 0 degrees and 19 minutes which means your luck is in, you don’t need to make any changes. If you are in Lowestoft at 52.4759646 latitude and 1.6905025 longitude, the magnetic declination is 2 degrees and 45 minutes west of grid north so you will want to account for this.

Remember for westerly magnetic declination, we add degrees so set your compass as you normally would and then, once you have your bearing, add two degrees to it and then your compass is aligned correctly to account for magnetic north.