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We use the continuity equation to derive a method for measuring the patternspeed of the Milky Way's bar/bulge from proper motion data. The method hasminimal assumptions but requires complete coverage of the non-axisymmetriccomponent in two of the three Galactic coordinates. We apply our method to theproper motion data from a combination of Gaia DR2 and VISTA Variables in theVia Lactea (VVV) to measure the pattern speed of the bar as$\Omega_\mathrm{p}=(41\pm 3)\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ (where the erroris statistical). This puts the corotation radius at$(5.7\pm0.4)\,\mathrm{kpc}$, under the assumptions of the standard peculiarmotion of the Sun and the absence of non-axisymmetric streaming in the Solarneighbourhood. The obtained result uses only data on the near-side of the barwhich produces consistent measurements of the distance and velocity of thecentre of the Galaxy. Addition of the data on the far-side of the bar pulls thepattern speed down to $\Omega_\mathrm{p}=(31\pm1)\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ but requires a lower transverse velocity forthe Galactic centre than observed. This suggests systematics of$5-10\,\mathrm{km\,s^{-1}kpc^{-1}}$ dominate the uncertainty. We demonstrateusing a dynamically-formed bar/bulge simulation that even with the limitedfield of view of the VVV survey our method robustly recovers the pattern speed.

The pattern speed of the Milky Way bar from transverse velocities