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Calibration

The process of calibration translates the measured magnetic vector into calendar years. A record of how the Earth's magnetic field has changed over time is required to do this, and is referred to as a secular variation or a calibration curve. A date is obtained by comparing the mean magnetic vector, defined by the declination and inclination values, with the secular variation curve; the potential age of the sampled feature corresponds to the areas where the magnetic vector overlaps with the calibration curve. Unfortunately, the Earth's magnetic poles have reoccupied the same position on more than one occasion, and can result in multiple age ranges being produced. Alternative chronological information is required in these situations to identify the archaeological significant age range. 

The current British secular variation curve was produced by Zananiri et al. (2007) using the RenCurve software developed by Lanos (2004). A calibrated date is obtained using the separate inclination and declination calibration curves. Probability distributions are produced for the calibrated inclination and declination values, before they are statistically combined to produce a single age estimate.

Probability distributions for the Declination and Inclination values.

Combination of the inclination and declination values in the RenDate programme.

Images redrawn by D. Bashford from the graphic outputs produced using the RenDate software (Lanos 2004). Image description:

  • (top left) The probability distributions produced for an inclination value of 72 degrees with an alpha-95 value of 3 degrees.
  • (top right) The probability distributions produced for a declination value of 12 degrees with an alpha-95 value of 3 dregrees.
  • (bottom) The probability distributions produced for the inclination and declination values are statistically combined (coloured in grey).