Context : The Sagittarius dwarf Spheroidal Galaxy ( Sgr dSph ) provides us with a unique possibility of studying a dwarf galaxy merging event while still in progress .
Moving along a short-period , quasi-polar orbit in the Milky Way Halo , Sgr dSph is being tidally dispersed along a huge stellar stream .
Due to its low distance ( 25 kpc ) , the main body of Sgr dSph covers a vast area in the sky ( roughly 15 \times 7 degrees ) .
Available photometric and spectroscopic studies have concentrated either on the central part of the galaxy or on the stellar stream , but the overwhelming majority of the galaxy body has never been probed .

Aims : The aim of the present study is twofold .
On the one hand , to produce color magnitude diagrams across the extension of Sgr dSph to study its stellar populations , searching for age and/or composition gradients ( or lack thereof ) .
On the other hand , to derive spectroscopic low-resolution radial velocities for a subsample of stars to determine membership to Sgr dSph for the purpose of high resolution spectroscopic follow-up .

Methods : We used VIMOS @ VLT to produce V and I photometry on 7 fields across the Sgr dSph minor and major axis , plus 3 more centered on the associated globular clusters Terzan 7 , Terzan 8 and Arp 2 .
A last field has been centered on M 54 , lying in the center of Sgr dSph .
VIMOS high resolution spectroscopic mode has then been used to derive radial velocities for a subsample of the observed stars , concentrating on objects having colors and magnitudes compatible with the Sgr dSph red giant branch .

Results : We present photometry for 320,000 stars across the main body of Sgr dSph , one of the richest , and safely the most wide-angle sampling ever produced for this fundamental object .
We also provide robust memberships for more than one hundred stars , whose high resolution spectroscopic analysis will be the object of forthcoming papers . Sgr dSph appears remarkably uniform among the observed fields .
We confirm the presence of a main Sgr dSph population characterized roughly by the same metallicity of 47 Tuc , but we also found the presence of multiple populations on the peripheral fields of the galaxy , with a metallicity spanning from [ Fe/H ] =-2.3 to a nearly solar value .

Conclusions :