LHCb is one of the four main experiments located at the Large Hadron Collider (LHC) at CERN, and has collected about 3 fb-¹ of proton-proton collisions at √s = 7 TeV and 8 TeV between December 2009 and December 2012. Designed for the study of B-meson decays and for precision CP-violation measurements, the LHCb detector requires a high resolution vertex reconstruction, a precise measurement of the charged particle’s momentum and an excellent particle identification. In this thesis, a study of the LHCb magnetic field map and two physics analyses are presented. Based on the magnetic field measurements collected during a dedicated campaign in February 2011, the magnetic field map is corrected for mis-alignments, considering global translations and rotations. A more reliable mapping of the field is provided, and is used for the LHCb event reconstruction since June 2011. As a consequence of this study, the mass resolution is improved, and a better agreement between the software alignment and the survey measurements of the tracking stations is obtained [...]