Geographic Correction of Archeomagnetic Data

Archeomagnetically determined values of inclination and declination are commonly reduced to a base location, the regional geomagnetic field being approximated either by the axial dipole or by a virtual dipole. We investigate the error in these approximations by using the spatial variations of the 1965 IGRF to simulate the temporal secular variation. The virtual dipole approximation is found to be significantly more accurate than the axial dipole approximation, particularly when the archeomagnetic site is at the same latitude as the base. Inclination error and declination error each have a normal distribution, but the directional dispersion is not circular-normal (Fisherian). Tables are presented by which the statistical error in reduction of archeomagnetic directions to a base site can be readily estimated.