Historically, in navigating long distances, the compass was as important an instrument for indicating direction as the sandglass was for marking time. However, until the 17th century, everyone believed that magnetic north coincided with true north, and mariners were unaware of this variation (see Geomagnetism, history of ); consequently, as they sailed west they found their position did not correspond with their location according to charts. Having learned that true north differed from magnetic north, instrument makers in some of the northern countries produced compasses in which the compass card was mounted on the magnetic needle in alignment with the amount of magnetic variation. Thus, while the needle pointed to magnetic north, the fleur‐de‐lison the compass card indicated true north. By this arrangement the compass had a built‐in correction for the magnetic variation. This was fine, as long as voyages were limited to regions where the amount of variation did not appreciably...
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Mandea, M. (2007). Main Field Maps. In: Gubbins, D., Herrero-Bervera, E. (eds) Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-4423-6_209
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