  We present radio polarimetric observations of a test region for the Southern
Galactic Plane Survey (SGPS), a radio survey in the Galactic plane in HI and
polarized continuum at 1.4 GHz with the Australia Telescope Compact Array
(ATCA) and the Parkes 64m dish. Ubiquitous structure in linearly polarized
intensity is often uncorrelated with structure in total intensity, indicating
Faraday rotation and depolarization. The one-dimensional second order structure
function of rotation measure for different position angles of the distance lag
shows a very shallow slope of the structure function on small scales and an
anisotropy in the slope on large scales. The anisotropy on large scales is
indicative of a gradient or large filament across the field. The shallow slope
can be explained by contributions of two separated Faraday screens. Assuming
that the two Faraday screens are the local and Carina spiral arms, the
structure function suggests an outer scale of structure in the local arm of
about 2 pc. This scale is similar to the typical scale of a Stroemgren sphere
of a B1-2 star. Therefore, we suggest that HII regions possibly dominate the
scale of structure in spiral arms.
