  The Galactic magnetic field is important in the dynamics of our Galaxy. It is
believed to play a role in star formation and influence the structure of the
Galaxy. In order to understand how the Galactic magnetic field originally
formed or how it is evolving, we must first determine its present topology. To
this end, we have used observations from the Canadian Galactic Plane Survey
(CGPS) to calculate the highest source density of rotation measures (RM) to
date in the disk of the Galaxy. Using these data, we estimate the Galactic
longitude of the RM null point in the outer Galaxy (where the RMs of
extragalactic sources are observed to pass through zero, on average, with
increasing Galactic longitude). We have also examined the RM scale height using
the CGPS latitude extension. The values of these parameters offer critical
constraints for modeling the large-scale magnetic field in the Galactic disk.
