  We present a new method that derives both velocity components in the
equatorial plane of a barred stellar disc from the observed line-of-sight
velocity, assuming geometry of a thin disc. The method can be applied to large
departures from circular motion, and does not require multipole decomposition.
It is based on assumptions that the bar is close to steady-state (i.e. does not
evolve fast), and that both morphology and kinematics are symmetrical with
respect to the major axis of the bar. We derive the equations used in the
method, and analyze the effect of observational errors on the inferred velocity
fields. We show that this method produces meaningful results via a simple toy
model. We also apply the method on integral-field data of NGC 936, for which we
recover both velocity components in the disc. Knowing both velocity components
in the disc, i.e. the non-observable transverse velocity in addition to the
line-of-sight velocity, puts additional constraints on dynamical models and
allows for new ways of determining parameters that are crucial in
characterizing galaxies.
