I. Apples to apples $A^2$: realistic galaxy simulated catalogs and
  photometric redshift predictions for next-generation surveys
  We present new mock catalogues for two of the largest stage-IV
next-generation surveys in the optical and infrared: LSST and Euclid, based on
an N-body simulation+semi-analytical cone with a posterior modification with
\texttt{PhotReal}. This technique modifies the original photometry by using an
empirical library of spectral templates to make it more realistic. The
reliability of the catalogues is confirmed by comparing the obtained
color-magnitude relation, the luminosity and mass function and the angular
correlation function with those of real data.
  Consistent comparisons between the expected photometric redshifts for
different surveys are also provided. Very deep near infrared surveys such as
Euclid will provide very good performance ($\Delta z/(1+z) \sim 0.025-0.053$)
down to $H\sim24$ AB mag and up to $z\sim3$ depending on the optical
observations available from the ground whereas extremely deep optical surveys
such as LSST will obtain an overall lower photometric redshift resolution
($\Delta z/(1+z) \sim 0.045$) down to $i\sim27.5$ AB mag, being considerably
improved ($\Delta z/(1+z) \sim 0.035$) if we restrict the sample down to
i$\sim$24 AB mag. Those numbers can be substantially upgraded by selecting a
subsample of galaxies with the best quality photometric redshifts. We finally
discuss the impact that these surveys will have for the community in terms of
photometric redshift legacy.
  This is the first of a series of papers where we set a framework for
comparability between mock catalogues and observations with a particular focus
on cluster surveys. The Euclid and LSST mocks are made publicly available in
the following link: http://photmocks.obspm.fr/.
