  In ghost imaging scheme, an illuminated light is split into test and
reference beams which pass through two different optical systems respectively
and an image is constructed by the second-order correlation between the two
light beams. Since both the two light beams are all diffracted when passing
through the optical systems, spatial resolution of ghost imaging is in general
lower than that of a corresponding conventional imaging system. When
Gaussian-shaped light spots are used to illuminate an object, randomly scanning
across the object plane, in ghost imaging scheme, we show that by localizing
central positions of the spots of the reference light beam, the resolution can
be enhanced by a factor of $\sqrt{2}$ same as that of the corresponding
conventional imaging system. We also find that the resolution can be further
enhanced by setting an appropriate threshold to the bucket measurement of ghost
imaging.
