The effects of applied 3-dimensional magnetic error fields on magnetic surfaces are measured in both the Joint European Torus (JET) and the Mega Ampére Spherical Tokamak (MAST). Static perturbation fields with various amplitudes have been applied in both devices using non-axisymmetric coils producing a dominantly n = 1 magnetic field to systematically perturb the plasma flux surfaces. The displacement of the plasma boundary is found to increase linearly with the strength of the applied field, and agrees well with predictive modelling which superimposes vacuum non-axisymmetric fields on a 2d equilibrium.