  In the present-day panorama of large spectroscopic surveys, the amount,
diversity, and complexity of the available data continuously increase. We
present a comprehensive catalogue, the Survey of Surveys (SoS), built by
homogeneously merging the radial velocity (RV) determinations of the largest
ground-based spectroscopic surveys to date, such as APOGEE, GALAH, Gaia-ESO,
RAVE, and LAMOST, using Gaia as reference. We have devised a multi-staged
procedure that includes: i) the cross match between Gaia and the spectroscopic
surveys using the official Gaia cross-match algorithm, ii) the normalization of
uncertainties using repeated measurements or the three-cornered hat method,
iii) the cross calibration of the RVs as a function of the main parameters they
depend on (magnitude, effective temperature, surface gravity, metallicity, and
signal-to-noise ratio) to remove trends and zero point offsets, and iv) the
comparison with external high-resolution samples, such as the Gaia RV standards
and the Geneva-Copenhagen survey, to validate the homogenization procedure and
to calibrate the RV zero-point of the SoS catalogue. We provide the largest
homogenized RV catalogue to date, containing almost 11 million stars, of which
about half come exclusively from Gaia and half in combination with the
ground-based surveys. We estimate the accuracy of the RV zero-point to be about
0.16-0.31 km/s and the RV precision to be in the range 0.05-1.50 km/s depending
on the type of star and on its survey provenance. We validate the SoS RVs with
open clusters from a high resolution homogeneous samples and we provide median
RVs for 532 clusters recently discovered by Gaia data. The SoS is publicly
available, ready to be applied to various research projects, such as the study
of star clusters, Galactic archaeology, stellar streams, or the
characterization of planet-hosting stars, to name a few.
