A compass does not point to the true North Pole but to direction that is a function of the North Magnetic Pole and the local secular variation to yield a magnetic declination.
There are also differences in the way the lichen is measured; while some suggest that the largest diameter should be measured, other scientists prefer the diameter of the largest inscribed circle.
A problem in dating lichens is the fact that several thalli can fuse together, making several minor lichens appears as a larger one of older age.
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials.
1.5 The Quaternary: Stratigraphic Framework and Terminology.
1.4 Atomic Structure, Radioactivity and Radiometric Dating.