The measurement of the safety factor q in tokamaks, which describes the winding of the helical magnetic field lines, is very important especially for the achievements of advanced scenarios. The motional Stark effect diagnostic can provide a direct measurement of the magnetic field orientation but the derivation of the q-profiles requires a simulation of the magnetic equilibrium taking into account inputs from several other diagnostics. This analysis can be affected by large errors. In order to validate the results, q profiles are compared with the radii of MHD modes, which can be attributed to surfaces of known q. This paper analyses the errors in the assumptions on the derivation of the MHD mode localization.