It has been found that the activation field for 180° domain wall motion in a BaTiO3 crystal is increased when the wall intersects an ``a'' domain for the case where the wall is parallel to the edge of the ``a'' domain. It is shown that this increase is only an apparent increase due to a reduction of the mean field E2 acting on the wall in the ``a'' domain region compared with the field E1 acting on the walls which do not intersect ``a'' domains. If d1=crystal thickness and d2=``a'' domain thickness, then E1/E2 = (d1+d2)/d1. As a result, it is calculated that the contribution of any ``a'' domain spikes in the surface to the increased surface-layer activation field is negligible. It is also shown that the motion of 180° walls is determined by the fields acting throughout the whole of the crystal and not just by the fields at the surface of the crystal.