Friday, 12 June 2015

Measuring magnetic history at Godrevy Point ... by Anthony Swan

Even in these days of the ubiquitous handheld GPS, those of you who regularly take to the hills and moors across the UK will be aware that a map and compass are essential for navigating you safely home.  Experienced walkers will know the importance of magnetic variation and how it can change from location to location, but will say that it is always west of Grid North. Well historically (since the production of OS maps) yes, but now in the very south-west corner of the UK this can no longer be said to be true.

How can this be? Surely your compass always points in the same direction? Well not exactly, you see most people think of the Earth's magnetic field as a traditional bar magnet, the type you used in physics class with a north & south pole at each end (something we call a dipole), but as my colleague Sue explained in an excellent blog last year this is not strictly true, the field is more complicated than that. It varies in both strength and direction depending on where you are on the Earth and your compass will always align itself with the local magnetic field. To complicate matters further, this field also varies over time, for example the field across the UK currently has an easterly movement of 10-13 arc minutes (0.17-0.27°) per year. It is this slow easterly movement of the field that now sees that south-west corner of the UK with an easterly Grid Magnetic Angle (GMA).

How do we know this? Well, people have been measuring and recording the magnetic field for hundreds of years. From early naval explorers who soon realised that as they circumnavigated the globe their compasses didn't always point north, through to scientists on Captain Scott's ill-fated Antarctic expedition, to us today at the British Geological Survey (you can find out more about us and our historical archives at this address:

How do we measure the field? Well that's something we at the BGS are very good at, in fact we in the UK are considered world leaders in what we do. And what we do takes us to some remote and interesting places around the world, from as far south as the Antarctic region to the very north of Alaska, with some sunny places in between.

The BGS run a series of magnetic observatories both in the UK and around the world, which allows us to produce very accurate global models of the Earth's magnetic field. However these observatories are spaced too far apart for us to accurately model the local magnetic variations across the UK that hill-walkers and ramblers rely on (we have three UK observatories located in the UK: Shetland Islands, Dumfries & Galloway & north Devon).

To produce the regional variation values that appear in your Ordnance Survey maps we run a programme of spot measurements across the UK every summer. We have a total of 41 of these so-called repeat stations across the UK, visiting each one every four years.

If you're thinking that it would be interesting to head down to one of these stations and take a look, I'm afraid that you may be a little disappointed. There's nothing actually there to indicate their presence, no markers, no trig-points, just open fields.

Each time we occupy a measurement site there are a certain number of tasks we need to carry out before we measure the magnetic field. Firstly we have to find the site location, we now use a modern GPS system that can give us our location to millimetre accuracy, however in the recent past the only way to find the site would be take measurements from recorded landmarks (which sometimes wasn't easy, as some of those landmarks had a habit of disappearing in the four years since the last visit).

Once the site has been found, we walk across the site area with a device that measures the full value of the magnetic field. This device doesn't give us the direction of the field, but it indicates its strength and the site survey will tell us if anyone has contaminated our measurement location with magnetic debris since we last visited (someone might have decided to bury a cable or metal waste nearby).

Once we're confident that we've found the measurement location and that the site is magnetically clean, we then have to determine the direction of geographic (or true) north. To do this we have a couple of methods - we can either use the differential GPS system or we can use a north seeking gyroscope. Either method allows us to determine the direction of true north from our measurement site with an accuracy of better than 10 arc seconds (0.0027°).

After obtaining our true north direction (azimuth) we take a series of measurements throughout the day using a survey theodolite (a device that can very accurately measure angles) with a fluxgate magnetometer fitted on top (this magnetometer can be thought of as a digital compass, but can measure the field much more accurately than a standard field compass).

At any point on the Earth the magnetic field can be thought of as a 3-dimensional vector, that is to say that it is made up of both horizontal (northerly and easterly) and vertical components. You may not know that your standard field compass is set up to compensate for the force of this vertical component. If however you decide to take your Northern Hemisphere compass on holiday to say Chile, you'll find it useless as the vertical component of the magnetic field in the Southern Hemisphere is in the opposite direction and your compass needle will no longer be balanced.

Our measurements throughout the day are essentially aligning the theodolite perpendicular to both the vertical and horizontal components of the magnetic field and very accurately recording the angles. Whilst making the angular measurements we are also running an instrument that measures the strength of the field (the one we used for the site survey). By combining the angle and strength measurements we can then use simple trigonometry to resolve the 3D vector of the magnetic field at that location.

Before we use this data in our regional models and pass it on to the Ordnance Survey for use in their maps we have to do one more thing. You see, as well as the magnetic field varying slowly over time (something we call secular variation), it also varies throughout the day due to the heating effect of the sun in the upper ionosphere (diurnal variation). The effects of space weather can also dramatically change the magnetic field throughout the day (you can find out more about this from our web-pages). These short-term variations have to be removed from our data before we pass it on to the Ordnance Survey and to do this we use our observatory data to determine the value of these variations at each site at the time of the measurements.

The values we record on site give us the declination angle at that particular location (that is the angle between true north and local magnetic north). We then produce a model of declination for the UK and correct it for grid north before supplying the Ordnance Survey with Grid Magnetic Angle (GMA) values for across the UK.
So why are we measuring magnetic history? Well, at the beginning of June this year, on a very wet and windy Cornish day we visited our repeat station at Godrevy Point (GOD on map). The measurements performed on this day were the first time in over 350 years that anyone has measured an easterly GMA across the UK, and it's something we at the BGS are very excited about.

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