  In this paper we compute the quantity which is truly measured in a large
galaxy survey. We take into account the effects coming from the fact that we
actually observe galaxy redshifts and sky positions and not true spatial
positions. Our calculations are done within linear perturbation theory for both
the metric and the observer velocities but they can be used for non-linear
matter power spectra. We shall see that the complications due to the fact that
we only observe on our background lightcone and that we do not truly know the
distance of the observed galaxy, but only its redshift is not only an
additional difficulty, but even more a new opportunity for future galaxy
surveys.
