Detector description of the ATLAS muon spectrometer and H8 muon testbeam

The Muon Spectrometer of the ATLAS experiment is a large and complex system of gaseous detectors. The simulation and the reconstruction of muon events require a careful description of these detectors, which either participate in the trigger or in the precision measurements of tracks. A thorough description of the passive materials, such as the toroidal magnet systems, is also needed to account for Coulomb scattering and energy losses. The operation of the muon spectrometer relies on the alignment of its precision chambers, so the geometrical model must fully implement their misalignments and deformations. We present the Detector Description chain employed in the Muon system and its integration in the software framework. The Muon Detector Description has been used successfully in the context of the Data Challenges, where it provides a unique and coherent geometry source for the simulation and reconstruction algorithms. It has also been validated in the context of the experimental program of the H8 testbeams, where analyses of the treatment of chamber alignment in track reconstruction relies crucially upon the detector description model.