Note atan2 as possible angular transformation method; have found more reliable if noise is present

doc/manual.tex

@@ -391,6 +391,8 @@ changes in $\theta$.
 Given an angular feature $\langle \alpha_1, \alpha_2 \rangle$ we can
 recover the original angle $\theta$ (in radians) by taking the
 $cos^{-1}(\alpha_2)$ and multiplying by the sign of $\alpha_1$.
+Another method uses the common 'atan2' function as
+$atan2(\alpha_1, \alpha_2)$.
 
 \begin{figure*}[htp]
   \centering