are not observed at all, and were removed from the solution and subsequent statistics. As shown in Fig. \[fig:maps-results-DT\], the gaps cause considerable inhomogeneity in the sky coverage and formal errors. The white lines and wedges in the left panel show the areas most affected by the data gaps. As expected, stars in those areas also have large formal errors, as seen in the right panel. The formal errors are plotted on the same scale as Fig. \[fig:maps-results\], panel [**c**]{}, to show that only the areas affected by dead time suffer from larger errors. 441 pixels are saturated, all of these corresponding to areas affected by dead time. For 3.5% of the sky the formal errors are larger than 3 mas. Globally, the parallax and position errors are about 16% higher than in the solution without data gaps. The proper motion errors, which are dominated by the errors in the [*Tycho*]{} positions, are less affected.

Solution without [Hipparcos]{} parallax prior \[sec:noparallaxprior\]
---------------------------------------------------------------------

For validation purposes it is desirable to compare the parallax values of the TGAS solution with [Hipparcos]{}. This is problematic when using the nominal TGAS since it already incorporates the [Hipparcos]{} parallaxes as a prior. As shown in 