  Diagnosing free electron laser (FEL) polarization is critical for
polarization-modulated research such as x-ray free electron laser (XFEL)
diffraction imaging and probing material magnetism. In an electron
time-of-flight (eTOF)\ polarimeter, the flight time and angular distribution of
photoelectrons were designed based on x-ray polarimetry for on-site diagnosis.
However, the transverse position of x-ray FEL pulses introduces error into the
measured photoelectron angular distribution. This work thus proposes a method
to monitor the transverse position using an eTOF polarimeter and explains how
to compensate for the error due to transverse position. A comprehensive
numerical model is developed to demonstrate the feasibility of the compensation
method, and the results reveal that a spatial resolution of 20 \(\mu\)m and a
polarity improved by 0.5\% is possible with fully polarized FEL pulses. The
impact of FEL\ pulses and a method to calibrate their linearity is also
discussed.
