Polar grid

\begin{matrix} (r, θ) \to (x, y) & (x, y) \to (r, θ) \\ \begin{matrix} x & = & r cos θ \\ y & = & r sin θ \end{matrix} & \begin{matrix} r^{2} & = & x^{2} + y^{2} \\ tan θ & = & \frac{x}{y} \end{matrix} \end{matrix}