  Magnetic field strengths in objects ranging from HII regions to cosmological
large scale structure can be estimated using dense grids of Rotation Measures
(RMs) from polarized background radio structures. Upcoming surveys on the SKA
and its precursors will dramatically increase the number N of background
sources. However, detectable magnetic field strengths will scale only as
$t^{-0.15}$, for an integration time $t$ on a fixed area of sky, so the
analysis techniques need to be optimized. A key factor is the difference in the
dispersion of intrinsic RMs for different populations, which must be carefully
accounted for to achieve the scientifically needed accuracies.
