  We present Rotation Measures (RM) of the diffuse Galactic synchrotron
emission from the Canadian Galactic Plane Survey (CGPS) and compare them to RMs
of extragalactic sources in order to study the large-scale reversal in the
Galactic magnetic field (GMF). Using Stokes Q, U and I measurements of the
Galactic disk collected with the Synthesis Telescope at the Dominion Radio
Astrophysical Observatory, we calculate RMs over an extended region of the sky,
focusing on the low longitude range of the CGPS (l=52deg to l=72deg). We note
the similarity in the structures traced by the compact sources and the extended
emission and highlight the presence of a gradient in the RM map across an
approximately diagonal line, which we identify with the well-known field
reversal of the Sagittarius-Carina arm. We suggest that the orientation of this
reversal is a geometric effect resulting from our location within a GMF
structure arising from current sheets that are not perpendicular to the
Galactic plane, as is required for a strictly radial field reversal, but that
have at least some component parallel to the disk. Examples of models that fit
this description are the three-dimensional dynamo-based model of Gressel et al.
(2013) and a Galactic scale Parker spiral (Akasofu & Hakamada 1982), although
the latter may be problematic in terms of Galactic dynamics. We emphasize the
importance of constructing three-dimensional models of the GMF to account for
structures like the diagonal RM gradient observed in this dataset.
