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Measurements have been made of the Earth's magnetic field more or less continuously since about 1840. Some measurements even go back to the 1500s, for example at Greenwich in London. If we look at the trend in the strength of the magnetic field over this time (for example the so-called 'dipole moment' shown in the graph below) we can see a downward trend. ...


You can use the total field intensity of an igrf model, or you can use the average value of all of the readings of the base station mag. Either way, don't change the value and don't move the base station during the survey.

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