  A new method for measuring deviations from axisymmetry in the velocity fields
of gas disks is presented. The method is an extension of the method that Franx,
van Gorkom and de Zeeuw developed for a single orbit gas ring. The measurement
is based upon a higher order harmonic expansion of the full velocity field.
Epicyclic theory is used to calculate the effects of a perturbation in the
potential on these harmonic terms. It is shown how the s_1 and s_3 harmonics
can be used to measure epsilon_R sin(2 phi_{obs}), where epsilon_R is the
elongation of the potential and phi_{obs} is an (unknown) viewing angle. As a
demonstration, I applied this method to the HI velocity fields of two spiral
galaxies, NGC 2403 and NGC 3198.
