3
Dec
2015
1

Map reading skills: What is magnetic north?

Mag_diagram_Eng_GMT

Magnetic North is on the move

The final part of our six-week blog series on map reading skills, focuses on magnetic north. Map reading is an essential skill for any explorer or outdoor enthusiast. We’ve teamed up with Steve Backshall to record a series of videos to remind you of the basics and help you feel confident with your map.

 

Hear what Steve has to say about magnetic north in this short video:

As Steve said, to fully get to grips with your compass and navigating, we also need to understand the three different norths that there are. Have a look at the legend on your Ordnance Survey map and you will see that there is a section called North Points. When it comes to compass navigation – this is an important section of the legend.

There are three norths. The first is true north – this is where the north pole is and for the purposes of navigation can be ignored. The next one is grid north – this is the top of your map. It’s the same for all Ordnance Survey maps which means that rather than being one single point – it is a line, as if the world is flat and all along the top is north. The third and final north is magnetic north. This is where the compass needle points to and is governed by the magnetic field of the earth. One thing that many people don’t realise when they’re new to outdoor walking and navigation is that their compass doesn’t point to grid north – except by coincidence in some areas. The compass needle is attracted by magnetic force, which varies in different parts of the world and is constantly changing.

This is where you need to refer to the map legend – in the North Points section it will tell you, at the time the map was printed, what the magnetic variation is and how much it is changing, and in which direction, annually. For example it will say something like:

Magnetic north is estimated at 3.0 degrees west of grid north for 2015. Annual change is approximately 0.2 degrees east.

Rounding this up/down to the nearest whole degree – what this means for this example, from a map near Norfolk, is magnetic north is approximately 3.0 degrees to the west of grid north. When you come to take your grid bearing you will need to add these three degrees on to your compass bearing so that grid north and magnetic north are aligned.

 

Is magnetic north constant?

For many years, the magnetic variation throughout Great Britain has been a few degrees West of grid North with the amount of variation changing every year, moving gradually to the east. The number has decreased, and now in the far South West of Britain, the North on your compass lies to the East of the North on your map for the first time since before OS even formed in 1791.  The change will slowly cross the country, but for now can only be appreciated in our Custom Made maps centred in the west of Cornwall.  Buy one now and you will find a new icon we have created in the legend to show the new relationship between the three Norths (magnetic, grid and true).

We show magnetic north on all of our maps (and state the date it was calculated), but for now only Custom Made will be showing the latest figures, which we obtain from the British Geological Survey each year. Assuming the shift in magnetic variation continues at the same rate, that handy adage, “Grid to Mag add, Mag to Grid get rid”, will cease to be a help and we’ll need to come up with a new one. Until now you could rely on adding the magnetic variation on if converting from a bearing taken on a map to one you want to use to use to explore in Britain.  But once magnetic north shifts to the east of Grid North then the variation will need to be subtracted.

 

Summing up

Map reading is an essential skill for any explorer or outdoor enthusiast, but can seem really daunting if you haven’t looked at an OS map since your Geography GCSE! To help you to get the most out of your map, and to #GetOutside to explore the Great British countryside, we teamed up with Steve Backshall and recorded a series of videos. They take you through the basics step by step and we’ll be showcasing Steve’s top tips to help you feel confident with your map.

Watch the full video playlist here:

Go back over our blog series and watch each video and our blog guide:

Download our map reading booklet or map reading made easy peasy booklet (aimed at children) for a handy reminder of the whole series.

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4 Responses

  1. Stuart Hall

    I have always understood westerly deviations (magnetic, longitude, and time) to be positive and easterly to be negative. The adage, “Grid to Mag add, Mag to Grid get rid” will therefore always work correctly.

    I find this to be intuitive, others may not I suppose.

  2. I am a keen rambler and looking for further adventures into the wild. I have enjoyed Steve’s video series and found them fun, quick and informative, however, I feel you should go further in explaining in greater detail the magnetic declination.

    On US maps in the key, they have a handy diagram they can use with a compass to add/subtract the variation, but in OS maps the diagram and explanation in the key does not provide enough information or maybe for a novice like me, i just simply don’t understand if i should add/subtract and by how much.

    I have read the pages on your site about it and still don’t quite understand how to interpret this (without the use of online calculators) if i were to pick up any OS 1:25000 or 1:50000 map. Several different examples and live recording explanations would be fantastic!

    Thanks

  3. Jac

    None of your explanations (video and on this page) explain what the map legend is actually saying when it states e.g. “magnetic north is estimated at 1°38’ and 1°36’ west of grid north respectively”. Please can you either update your maps or correct the explanation? As it is the information you’ve provided is a waste of time for a novice.

  4. Gringo

    The blog series and video are a great idea to get people interested in map reading and navigation but I really think that OS, above all others, should be making sure that the correct terms are used, especially to avoid confusing novices.

    The difference between Grid North and Magnetic North is known as the Grid Magnetic Angle, and it is this that concerns us when using map and compass.

    Declination or Magnetic Variation is the difference between True North and Magnetic North, and is therefore of no concern whatsoever for map reading, but is used when using a chart with a lat / long graticule.

    Magnetic Deviation is the term for anything that prevents your compass needle from aligning itself with the Earth’s magnetic field: proximity to ferrous metals, powerlines, local magnetic anomalies etc.

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