  Pulsar Timing Arrays (PTAs) are expected to be able to detect gravitational
waves (GWs) from individual supermassive black hole binaries in the near
future. In order to identify the host galaxy of a gravitational wave source,
the angular resolution of PTAs should be much better than that expected from
the conventional methodology of PTAs. We study the potential usefulness of
precise pulsar-distance measurements in the determination of the sky location
of a single GW source. Precise distance information from external observations
such as astrometry by Very Long Baseline Interferometry is incorporated as
priors in the PTA analysis and we evaluate the precision of the sky location of
a GW source by simulating PTA data of 12 milli-second pulsars with only the GW
signal and the Gaussian white noise in the timing residuals. We show that only
a few pulsars with a distance precision of 1 pc will improve the precision of
the source location by more than 1 order in the presence of white noise of 10
ns.
