  The use of Reduced Proper Motion in identifying isolated white dwarfs has
long been used as a proxy for the absolute magnitude in a population with known
kinematics. This, however, introduces a proper motion detection limit on top of
the existing photometric limit. How the survey volume is hampered by this extra
parameter is discussed in Hambly et al. 2012. In this work, we discuss some
robust outlier rejection methods in order to minimise the proper motion limit
and hence maximise the survey volume. The generalised volume, corrected for the
distance of the Sun from the Galactic Plane, is integrated explicitly.
