  The Tremaine-Weinberg method is a well-known model independent technique for
measuring density wave pattern speeds in spiral galaxies. Here we show how it
can be applied to the data cubes (maps of surface brightness and velocity)
obtained in H-alpha emission using a Fabry-Perot spectrometer. One of the main
difficulties, the discontinuity of the H-alpha emission, is resolved using the
neighbouring stellar continuum delivered by the data cube. We argue from
symmetry that the motions not associated with the density wave should cancel.
We show that our pattern speeds are reasonable by computing corotation radii,
and comparing them to measured bar lengths. Simulations including star forming
gas also add credibility to our results. Nevertheless it will be necessary to
compare them with results using the spectra of the stellar components to
quantify any systematic deviations from valid pattern speed values.
