The secrets behind a compass: What are agonic and isogonic lines?

You may never have heard of agonic and isogonic lines, but they are the secret behind what your compass tells you, says Simon Phillips

Imagine for a moment, after a long expedition, you are at the geographic north pole. You look at your compass, but, perhaps unsurprisingly, it is far from pointing at your feet.

The needle in a compass is attracted towards the magnetic north pole, which happens to be on the move. Depending upon your point of view, this makes for something remarkably interesting or somewhat of an inconvenience, and anyone who has navigated with a paper chart is familiar with checking the compass rose for the correct amount of variation, plus or minus the correction for the years since the chart was published, to move between true and compass courses.

You may not have wrestled with the concept of variation much since sitting your theory exams, and at the moment, you can pretty well ignore it (in the UK at least), for reasons I’ll come to, but as time goes on, getting variation right is likely to become more and more important again when navigating with a compass.

For sailors, understanding the Earth’s magnetic field is important for safe and accurate navigation. Central to this understanding are what are known as agonic and isogonic lines. These play a crucial role in determining magnetic declination – the angle between true north and magnetic north at a given location.

The Compass

On your yacht, the compass will in theory point to magnetic north all by itself. However, there is another factor at play here called deviation. This is caused by onboard items that will alter the magnetic field locally on your boat – a list that includes the engine, navigation equipment, gas bottles, nearby wiring and, of course, a mobile phone running a navigation app placed next to the compass.

All these items will distort the reading on the compass to varying extents. While some exert a magnetic influence of their own, others merely disrupt the Earth’s magnetic field – an effect that will vary depending on the yacht’s heading.

We’ll put aside any considerations of the vessel’s deviation for the moment, as this is different on every vessel and must be compensated for accordingly by a compass adjuster. Instead, let’s talk a little more about where the compass points to and how this is different in various locations around the world.

True North vs Magnetic North

Lines of longitude are drawn on the charts from the (geographical) north pole to the south pole. True North is where these lines of longitude – known as meridians – all converge together in the north, forming the North Pole, which is in the Arctic Ocean.

Similarly, the same is true for the South Pole – the lines of longitude all converge at a single point, forming the South Pole, which is on the Antarctic landmass deep beneath the ice. It is around the axis connecting these two points that the Earth rotates and subsequently how angles of tilt are measured, seasons and daylight hours are known.

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If you wish to travel from the bottom of your chart to the top, you travel true north, following the lines of longitude. Your compass, however, points to magnetic north. If you used it to steer 000 degrees continually, you would eventually find the Earth’s magnetic North Pole, but not necessarily the top of your chart.

Currently, magnetic north is at 86º north and 142º east, approximately 280 miles south of the geographical North Pole in the depths of the Arctic Ocean north of Russia, having moved over 400 miles in the last 25 years.

If you consider that after 60 miles of sailing, a 1° course error will put you one mile away from your intended destination, you’ll see this can start to add up – especially when sailing long distances, such as a transatlantic.

The British Virgin Islands and Antigua both have beautiful beaches, clear waters and lovely anchorages, but nevertheless it is a wise idea to know where you are. On that crossing, you will also be moving through areas of greater and lesser variation, increasing the potential for error. Therefore it’s a good idea to know what variation you’re dealing with.

Furthermore, the positions of magnetic north and south move, keeping us on our toes. Over the last 150 years, the magnetic North Pole has moved approximately 620 miles, or around 1,000km. According to scientists, the magnetic pole migrates around 10km per year, although this increased to around 40km a year a few years ago. The poles can also, in theory, flip from pole to pole – but this is only thought to occur every 300,000 years, according to NASA.

The Earth’s Core

The reason why the magnetic pole changes and is constantly on the move is due to the composition of our planet. Earth’s core is composed of molten iron, the outer core of which moves and is like a huge magnet.

Although this magnetic field is relatively weak, it extends out into space, influencing things like solar radiation and the Aurora Borealis, as well as the needle or card of your compass.

At any given point on Earth’s surface, there are three main magnetic elements: declination, inclination, and intensity.

This angle varies depending on where the observer is located on the Earth’s surface. Inclination is the angle that the geomagnetic field is tilted with in relation to the surface of the Earth. This magnetic inclination varies from 90° (perpendicular to the surface) at the magnetic poles to 0° (parallel to the surface) at the magnetic equator. Finally, the intensity is the strength of the magnetism at any given point.

Declination

Magnetic declination is given by the difference and direction between the magnetic pole and true north. Depending upon where you are in the world, the direction and amount of declination differs based on how these two poles align relative to each other. When these two poles align, declination is zero and it is here that there is nothing to be done to convert magnetic to true because you are situated on what is known as an agonic line.

The first chart with declination lines on it is believed to be from 1701, made by Edmond Halley, a physicist, astronomer and mathematician, Fellow of the Royal Society, of comet fame which he observed in 1682.

Agonic lines

The word agonic is from the Greek for ‘no angle’. An agonic line is defined as ‘an imaginary line around the earth passing through both the north pole and the north magnetic pole, at any point on which a compass needle points to true north.’ It is, however, very far from a straight line in the way a Great Circle is, and snakes its way around the globe.

If you are west of this agonic line, your compass will point east of true north, giving a positive declination. Similarly, if you are east of the agonic line, the compass will point west of true north, giving a negative declination. As an example, in the English Channel the declination is almost zero, as of 2024, while in the mid-Atlantic it is over 16° W. This increases to up to 24° W in the Southern Atlantic Ocean.

Isogonic lines

All areas with constant magnetic declination are connected with isogonic lines. Much like isobars on a weather chart, these are magnetic ‘contour lines’ joining areas of the same magnetic declination. Isogonic lines are illustrated by the magenta lines drawn on small-scale charts in degrees and minutes either east or west. On larger-scale charts, you will find compass roses around the chart with more information – degrees and minutes, along with the annual increase of decrease in this figure.

These isogonic lines form curved lines across the globe, reflecting the irregularity of the Earth’s magnetic field. Where isogonic lines converge, declination is zero, and true north aligns with magnetic north. As we move away from an agonic line, the declination becomes either east or west.

There is currently an agonic line (green in the illustration) running through the UK, crossing the English Channel and entering the coast just east of Bridport in Dorset and exiting close to the Royal Northumberland Yacht Club in Blyth.

Anywhere along this line, you have no calculation to do to convert Magnetic to True as they are the same. Travelling east, you’ll find 1º E of declination running from Beachy Head in Sussex to Brancaster Staithe in Norfolk, while the 1º W isogonic line skirts Land’s End, clips Pembrokeshire, Anglesey and the Isle of Man, slicing Scotland in half and shooting off through the Orkneys.

Compass course corrections

In practical terms, knowledge of agonic and isogonic lines gives an awareness of how we need to apply calculations to convert magnetic to true for chartwork and in many respects, an idea of where we are in the world. This is important as it is how our navigation is affected.

Knowing the magnetic declination at any given location allows us to adjust the compass readings to find true north accurately. This is essential for plotting courses on the chart and determining bearings – for example, obtaining a position using a three-point fix using a hand-held compass and then knowing what to do to convert magnetic to true.

Having a picture in your mind’s eye of where the agonic and isogonic lines currently are at the start of each year could be an easy way to make quick interpolations of the right amount of variation to apply, without any fiddly mental arithmetic.

Hazards and route planning

Incorrectly accounting for magnetic declination would lead to navigation errors, resulting in the wrong course steered and potentially putting you in a position you do not want to be in. Understanding the variation in declination along the route, we can plan safe passages and avoid hazards.

Chartplotters

While many modern navigation systems, such as GPS, radar and chartplotters, rely on magnetic bearings for accurate positioning, it is not as easy to extract magnetic variation information from an electronic chart as it is a paper one. The settings of some plotters can, however, be changed to show magnetic or true north. If you use magnetic north, you can go straight from chart to compass, accounting only for deviation.

Serious errors

As magnetic declination varies from one location to another, for instance when crossing the Atlantic, it is important to be aware of these changes from one location to another. In the Canaries, the declination is approximately 3° W, and in Antigua it is approximately 15° W.

Therefore, after a couple of weeks of sailing in the sunshine, we gradually need to change the calculations to remain on course. The odd few degrees here and there can’t be accurately helmed, but a difference of 15° probably can.

In conclusion to this, understanding agonic and isogonic lines is important as they are fundamental concepts in magnetic navigation and essential for us to navigate accurately and safely across the world’s oceans.

By understanding the background and theory behind these lines, we can apply this knowledge to ensure precise navigation, efficient voyages, and the safety of our vessels and crew. With this understanding, we can confidently navigate the wonderful magnetic maze of our planet’s oceans, using the Earth’s magnetic field to chart a course with precision and skill.

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