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See https://stackoverflow.com/a/21486462/380384

Or follow these steps

1. Create two vectors $A=(S-E)$ and $B=(W-E)$
2. Calculate the dot product $$\cos\theta = A\cdot B = A_x B_x + A_z B_z$$
3. Calculate the magnitude of the cross product $$\sin\theta = |A \times B| = A_x B_y - A_y B_x $$
4. Use the ATAN2() function to get the included angle $$\theta = {\rm atan2}(\sin \theta, \cos \theta)$$
5. Get the desired angle by $\pi - \theta$