  Matter effects in neutrino propagation translate into effective parameters
for the oscillation and fake CP- and CPT-odd quantities, even in a scenario,
such as $\Delta_{12}=0$, where no genuine CP violation is present. This fact
seems to impose severe restrictions on the determination of intrinsic
parameters of the system from long-baseline experiments. We show, however, that
the resonance in the effective mixing $\tilde{\theta}_{13}$ can be observed for
a certain range of baselines. This provides a way to measure the vacuum mixing
angle $\theta_{13}$ and the sign of $\Delta m_{23}^2$ from atmospheric
neutrinos, using a detector with energy resolution and charge discrimination.
