  In this work, we research and evaluate multiple pose-graph fusion strategies
for vehicle localization. We focus on fusing a single absolute localization
system, i.e. automotive-grade Global Navigation Satellite System (GNSS) at 1
Hertz, with a single relative localization system, i.e. vehicle odometry at 25
Hertz. Our evaluation is based on 180 Km long vehicle trajectories that are
recorded in highway, urban and rural areas, and that are accompanied with
post-processed Real Time Kinematic GNSS as ground truth. The results exhibit a
significant reduction in the error's standard deviation by 18% but the bias in
the error is unchanged, when compared to non-fused GNSS. We show that the
underlying principle is the fact that errors in GNSS readings are highly
correlated in time. This causes a bias that cannot be compensated for by using
the relative localization information from the odometry, but it can reduce the
standard deviation of the error.
