Track resolution measurements for a time projection chamber with gas electron multiplier readout

We present measurements of track resolution from two gas electron multiplier TPC prototypes under study in Canada. The results are from measurements of cosmic rays using relatively large rectangular pads that rely on diffusion to spread the charge across more than one pad per row. Results with magnet fields up to 5.3 Tesla are shown and compared to Monte Carlo simulations. This represents the first demonstration of the tracking capabilities of a GEM TPC in magnetic fields.