Torus parametrization and inverse

[Jess_l]

I've been looking at the torus parametrization
\begin{equation}
\phi(u,v)=((r\cos u+a)\cos v, (r\cos u +a)\sin v, r\sin u)
\end{equation}
with \begin{equation}a>0, r\in(0,a)\end{equation}. I want to invert this map to get a chart map for the torus.
Can anyone give me a hand with this?
Thanks!

 

[mathwonk]

well, u seems to equal arcsin(z/r). how's that for a start?