  Experiments dedicated to the measurement of the electric dipole moment of the
neutron require outstanding control of the magnetic field uniformity. The
neutron electric dipole moment (nEDM) experiment at the Paul Scherrer Institute
uses a 199Hg co-magnetometer to precisely monitor magnetic field variations.
This co-magnetometer, in the presence of field non-uniformity, is responsible
for the largest systematic effect of this measurement. To evaluate and correct
that effect, offline measurements of the field non-uniformity were performed
during mapping campaigns in 2013, 2014 and 2017. We present the results of
these campaigns, and the improvement the correction of this effect brings to
the neutron electric dipole moment measurement.
