Computing main-field models derived from MAGSAT and ¿rsted satellite magnetic measurements, the crustal influence on satellite data can be treated as random noise. Satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non-core field at the observatory locations. Crustal biases for the horizontal northward X, eastward Y and vertical downward Z components are computed for all magnetic observatories where data are available for MAGSAT (1979--1980) and ¿rsted (1999--2000) epochs, to study their correlation over twenty years. For a set of observatories installed after 1979 new crustal biases are computed.