  Anisotropic transverse flow is one of the most important observables in the
study of ultra-relativistic nucleus-nucleus collisions. Detector acceptance
non-uniformity in the transverse plane introduces substantial bias in the flow
analysis dictating the need for specific corrections. The results of flow
analysis in Pb-Pb collisions at the beam energy of 40$A$~GeV recorded with the
fixed target experiment NA49 at the CERN SPS are presented. The three-subevent
technique is used for the differential measurements of the directed and
elliptic flow. Corrections for the detector acceptance anisotropy in the
transverse plane are applied using an extension of the Qn-Corrections Framework
developed originally for the ALICE experiment at the LHC. The results are
compared with those previously published by the STAR at RHIC and the NA49 at
CERN SPS collaborations. In the future, the developed procedure will be used
for the analysis of the new Pb-Pb data collected by the NA61/SHINE experiment
at the CERN SPS.
