By stacking residuals from a series of standard Tangent Plane astrometric fits based on 2MASS we can see (as in the example below) that there are no significant astrometric distortions over the whole field of view. The individual detector astrometric solutions achieve rms accuracies of around 70-80mas per star - generally dominated by rms errors in 2MASS stars. Even at high Galactic latitudes there are sufficient calibrators to give systematic residuals at the ~25mas level per detector. The global systematics from stacking multiple solutions are better than this as can be seen in the accompanying figure. A Tangent Plane projection (TAN) is being used for all data products.