  Polarised electron and positron beams are key ingredients to the physics
programme of future linear colliders. Due to the chiral nature of weak
interactions in the Standard Model - and possibly beyond - the knowledge of the
luminosity-weighted average beam polarisation at the $e^+e^-$ interaction point
is of similar importance as the knowledge of the luminosity and has to be
controlled to permille-level precision in order to fully exploit the physics
potential. The current concept to reach this challenging goal combines
measurements from Laser-Compton polarimeters before and after the interaction
point with measurements at the interaction point. A key element for this
enterprise is the understanding of spin-transport effects between the
polarimeters and the interaction point as well as collision effects. We show
that without collisions, the polarimeters can be cross-calibrated to 0.1 %, and
we discuss in detail the impact of collision effects and beam parameters on the
polarisation value relevant for the interpretation of the $e^+e^-$ collision
data.
