  Accurate characterization of thick disc properties from recent kinematic and
photometric surveys provides converging evidences that this intermediate
population is a sequel of the violent heating of early disc populations by a
merging satellite galaxy.
  The thick disc population is revisited under the light of new data in a
number of galactic sample fields. Various thick disc hypotheses are fitted to
observational data through a maximum likelihood technique. The resulting
characteristics of the thick disc are the following : a scale height of 760
$\pm$ 50 pc, with a local density of 5.6 $\pm$ 1 \% of the thin disc. The scale
length is constrained to be 2.8 $\pm$ 0.8 kpc, well in agreement with the disc
scale length (2.5 $\pm$ 0.3 kpc). The mean metallicity of the thick disc is
found to be -0.7 $\pm$ 0.2 dex, with no {significant} metallicity gradients.
  These photometric constraints in combination with kinematic data give new
constraints on the thick disc formation. We show that thick disc
characteristics are {hardly compatible with a top-down formation scenario} but
fully compatible with a violent merging event arising at the early thin disc
life time as described by Quinn, Hernquist \& Fullagar (1993).
