  Matter density uncertainties can affect the measurements of the neutrino
oscillation parameters at future neutrino factory experiments, such as the
measurements of the mixing parameters $\theta_{13}$ and $\deltacp$. We compare
different matter density uncertainty models and discuss the possibility to
include the matter density uncertainties in a complete statistical analysis.
Furthermore, we systematically study in which measurements and where in the
parameter space matter density uncertainties are most relevant. We illustrate
this discussion with examples that show the effects as functions of different
magnitudes of the matter density uncertainties. We find that matter density
uncertainties are especially relevant for large $\stheta \gtrsim 10^{-3}$.
Within the KamLAND-allowed range, they are most relevant for the precision
measurements of $\stheta$ and $\deltacp$, but less relevant for ``binary''
measurements, such as for the sign of $\ldm$, the sensitivity to $\stheta$, or
the sensitivity to maximal CP violation. In addition, we demonstrate that
knowing the matter density along a specific baseline better than to about 1%
precision means that all measurements will become almost independent of the
matter density uncertainties.
