  A new method for calibrating the hadron response of a segmented calorimeter
is developed. It is based on a principal component analysis of the calorimeter
layer energy deposits, exploiting longitudinal shower development information
to improve the measured energy resolution. Corrections for invisible hadronic
energy and energy lost in dead material in front of and between the ATLAS
calorimeters were calculated with simulated Geant4 Monte Carlo events and used
to reconstruct the energy of pions impinging on the calorimeters during the
2004 Barrel Combined Beam Test at the CERN H8 area. For pion beams with
energies between 20 and 180 GeV, the particle energy is reconstructed within 3%
and the energy resolution is improved by about 20% compared to the
electromagnetic scale.
