  We give the conversion equations which lead from experimental values of the
$3p \to 1s$ transition energy in pionic hydrogen and the total width of the
$1s$ level to values of the s-wave threshold scattering parameters for the
processes $\pi^-p \to \pi^-p$ and $\pi^-p \to \pi^0n$ respectively. Using a
three-channel potential model, we then calculate the electromagnetic
corrections to these quantities, which remove the effects of the Coulomb
interaction, the external mass differences and the presence of the $\gamma n$
channel. We give the s-wave scattering parameters obtained from the present
experimental data and these electromagnetic corrections. Finally we discuss the
implications for isospin invariance.
