The high angular resolution and dynamic range of VLA radio continuum mapping of planetary nebulae has raised expansion parallax measurement to the most sensitive method for determining PN distances (Hajian et al 1995). In order to derive the distance, the proper motion in the plane of the sky must be compared to the expansion velocity measured from emission line profiles. For a spherical PN with a measured line of sight expansion velocity, the comparison is simple. However most PN are not spherical, and their axisymmetric structures may be tilted to the line of sight; the line of sight expansion velocity cannot therefore be used for distance estimation from expansion proper motion. 3-D kinematic modelling is required in order to determine the expansion velocity in the plane of the sky.