  We compare maps of scattering polarization signals obtained from
three-dimensional (3D) radiation transfer calculations in a magneto-convection
model of the solar atmosphere using formal solvers based on the "short
characteristics" (SC) and the "long characteristics" (LC) methods. The SC
method requires less computational work, but it is known to introduce spatial
blurring in the emergent radiation for inclined lines of sight. For polarized
radiation this effect is generally more severe due to it being a signed
quantity and to the sensitivity of the scattering polarization to the model's
inhomogeneities. We study the differences in the polarization signals of the
emergent spectral line radiation calculated with such formal solvers. We take
as a case study already published results of the scattering polarization in the
Sr I $4607~\unicode{xC5}$ line obtained with the SC method, demonstrating that
in high-resolution grids it is accurate enough for that type of study. In
general, the LC method is the preferred one for accurate calculations of the
emergent radiation, reason why it is now one of the options in the public
version of the 3D radiative transfer code PORTA.
