The comparison of the Gaia catalogue to external Catalogues is a tricky task as the Gaia Catalogue is unique in many ways: it combines the spatial resolution of Hubble with a complete survey all over the sky in optical wavelength, down to a -magnitude 20, unprecedented astrometric accuracy and all-sky homogeneous photometric data. However, it is essential to establish the quality and reliability of the Gaia data, and the comparison with external catalogues is one way towards a deeper understanding of many of the parameters describing the performance of the Catalogue: overall sky coverage, spatial resolution, catalogue completeness and, of course, precision and accuracy of the different types of data for the various categories of objects observed by Gaia. A special attention is given to the detection of any possible bias, versus sky position, versus magnitude or colour. The objective of this section is to present the many tests made in these directions.
We describe here the compilation of catalogues that we made and how we compared them with Gaia data. The main results are summarized in Arenou et al. (2018).