I want to dicuss what large galaxy surveys which go out to redshift z=1 or more truly measure. I shall build my arguments, starting with the galaxy angular position and redshift and I shall derive the quantity to which this relates in first order perturbation theory. 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 in not only an additional difficulty, but especially a new opportunity for future galaxy surveys.

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