Magnetic Confusion

British Geological Society
BBC, April 17, 1999

Edinburgh Airport is repainting its runway strip markings after changes in the Earth's magnetic field rendered them inaccurate. The markings no longer tally with aircraft navigation systems because of a fractional shift in bearings. The British Geological Survey said after the change, magnetic north in the Scottish capital is around 5.5 degrees west of true geographic north. Geophysicist Dr Toby Clark said: "The precise thing to say is that this not because the runways have moved or changed direction in any way. "When the runways were built in the 1940s, aircraft would be coming down to land and their compasses would be showing a bearing of - rounded to the nearest 10 degrees - a bearing of zero, seven zero degrees. "But in the 50 years since the runways were built, the Earth's magnetic field has changed direction and now when they are coming in to land the magnetic compass, rounded to the nearest 10 degrees, will be showing zero, six zero degrees."

Dr Clark said the movement in the Earth's magnetic field has been measured in the UK for 150 years. The shift is due to changes in the flow of the Earth's molten iron core, where the Earth's magnetic field is generated. "From a navigational point of view these changes do have to be taken into account," said Dr Clark. Scientists have found the Earth's magnetic field has changed by between 20 and 25 degrees over the last 300 years. In geological history, the Earth's magnetic poles have completely swapped over many times. Dr Clark stressed that the use of magnetic headings on runways was mainly historical and aircraft positioning systems had rendered them more or less obsolete. However, the magnetic shift can still have an impact on some people, particularly map makers, hill walkers and those using small boats. Every airport has to change its markings on occasion to take account of the fractional magnetic changes. Edinburgh's main runway is being shut down for 10 nights during the repainting operation with flights transferred to the secondary runway. An airport spokesman said the only adverse impact might be a temporary increase in aircraft noise, for which he apologised.