Differences in position between the primary data set (TGAS) and the Hipparcos catalogue for 86 928 sources: a) difference in right ascension, (α_T−α_H)cosδ; b) difference in declination, δ_T−δ_H. Median differences at epoch J2015.0 are shown in cells of about 3.36 deg². c) Number of sources per cell used to compute the median differences here and in Fig. C.1. Some empty cells are shown in white. The position differences have been corrected for the orientation difference between the Hipparcos reference frame and the reference frame of Gaia DR1. The maps use an Aitoff projection in equatorial (ICRS) coordinates, with origin α = δ = 0 at the centre and α increasing from right to left.