Magnetic field strength and direction over the UK change both with time and according to location. Over the UK, declination varies by over 8 degrees from the most easterly to the most westerly points, and changes in time. Information published on Ordnance Survey maps needs to be regularly updated to take account of these changes. The data gathered from the re-occupation of a network of survey points scattered the length and breadth of the UK along with data from the three UK observatories, provides this information through annual updates of the UK Regional Geomagnetic Model. This is a model of the magnetic field of the UK and its secular variation, or slow rate of change in time. The accuracy of this model is dependent on the quality of the repeat station data and the regularity with which they are re-occupied.
The locations of the magnetic repeat stations are chosen for their isolation from magnetic interference. The survey position is fixed both by marker and by survey bearings in order that an exact re-occupation can be made during future surveys. The current network was established in Great Britain between 1985 and 1988 and from 2008 comprises 41 stations. The rolling programme of re-occupations means that all the repeat stations are visited at least once every four years.
The data gathered from the United Kingdom magnetic survey are also combined with similarly collected data from across the world in order to compute and assess global and continental-scale models of the geomagnetic field.
The angle between grid north (British National Grid) and magnetic north can be computed using the Grid Magnetic Angle Calculator.