Interpolation method for determining the magnetic field strength B and azimuth θ of the observed point M. This is achieved by determining the affine coordinates α₁ and α₂ of the AM vector in the affine basis defined by the AB and AC vectors. The points A, B, and C result from model values. A and B correspond to the same B₁ value (isogauss) of the magnetic field strength. A and C correspond to the same azimuth θ₁ value (isoazimuth) of the magnetic field.