  Results are presented for directed flow of negatively charged pions measured
relative to the spectator plane in Pb+Pb collisions at the beam energy 40A GeV
recorded by the NA49 experiment at CERN. The measure-ments were performed as a
function of rapidity and transverse momentum in two classes of collision
centrality. The projectile spectator symmetry plane is estimated using the
transverse segmentation of the forward hadron calorimeters. Analysis details
related to event selection and particle dentification are provided along with a
description of the procedure used to apply corrections for the limited
detection efficiency. Additionally a systematic study of effects due to
detector non-uniformity on pion directed flow measurement is presented.
