Approximately 51 000 observations made between 1955 and 1973 were used to produce the magnetic charts of Canada for 1975. A least-square method was used to derive sixth degree polynominals for the rectangular components X(north), Y(east), and Z(vertical). Cubic time terms were included to eliminate the customary laborious method of updating the data to the desired epoch.To reflect desired wavelengths of approximately 1000 km it was necessary to divide the map area of 31 × 106 km2 into quadrants with a unifying overlap of 10%. For consistency, X and Y were analysed together using Maxwell's curl-free relation (curl F)Z = 0. All data were weighted according to type and age.From the derived polynomials, values of D (magnetic declination), H (horizontal intensity), and Z were computed at 2° geographic grid intervals. These values were used to derive the final charts using a standard contouring package.The overall root mean square (rms) fit of the model to the input data is 174 nT. In the auroral zone, an area of high magnetic activity, the fit is generally poor. The fit to over 8500 supplementary observations is 170 nT, confirming the reliability of the model.