  The misalignment of field coils in tokamaks can lead to toroidal asymmetries
in the magnetic field, which are known as intrinsic error fields. These error
fields often lead to the formation of locked modes in the plasma, which limit
the lowest density that is achievable. The intrinsic error fields on MAST have
been determined by the direct measurement of the toroidal asymmetry of the
fields from these coils and have been parameterised in terms of distortions to
the coils. The error fields are corrected using error field correction coils,
where the optimum correction is found by determining the current required to
ensure that the discharge is furthest from the onset of a locked mode. These
empirically derived corrections have been compared with the known coil
distortions. In the vacuum approximation there is a factor of ~ 3 difference
between the predicted and empirically determined correction. When the plasma
response is included better agreement is obtained, but there are still some
cases where the agreement is not good, which suggests that other effects such
as the non-linear coupling of the error field to the plasma are important.
