  The Galactic magnetic field is an integral constituent of the interstellar
medium (ISM), and knowledge of its structure is crucial to understanding
Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources
have been the basis of comprehensive Galactic magnetic field models. Polarised
extended emission (XE) is also seen along lines of sight through the Galactic
disk, and also displays the effects of Faraday rotation. Our aim is to
investigate and understand the relationship between EG and XE RMs near the
Galactic plane, and to determine how the XE RMs, a hitherto unused resource,
can be used as a probe of the large-scale Galactic magnetic field. We used
polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near
1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis
Telescope. We calculated RMs from a linear fit to the polarisation angles as a
function of wavelength squared in four frequency channels, for both the EG
sources and the XE. Across the CGPS area, $55^{\circ} < {\ell} <193^{\circ},
-3^{\circ} < b < 5^{\circ}$, the RMs of the XE closely track the RMs of the EG
sources, with XE RMs about half the value of EG-source RMs. The exceptions are
places where large local HII complexes heavily depolarise more distant
emission. We conclude that there is valuable information in the XE RM dataset.
The factor of 2 between the two types of RM values is close to that expected
from a Burn slab model of the ISM. This result indicates that, at least in the
outer Galaxy, the EG and XE sources are likely probing similar depths, and that
the Faraday rotating medium and the synchrotron emitting medium have similar
variation with galactocentric distance.
