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I know its a little silly, but I got the wrong sign several times. Just to be clear, $z=r\cos(\phi), -\frac{\pi}{2}\leq\phi\leq\frac{\pi}{2}$ when converting from cartesian to spherical. So, how do I determine the sign? Thanks!

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    If $\phi$ here is supposed to be co-latitude, the version of spherical coordinates that I am accustomed to has it go from $0$ (North Pole) to $\pi$ (South Pole). It seems as if you're measuring from the equator instead of from the poles?2012-02-15
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    Yes, exactly. But the method would be the same with little changes, wouldn't it?2012-02-15
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    If, as you claim, the changes are "little", then you can figure out how to correct $\phi=\arccos\frac{z}{r}$, which assumes measurement from the poles, to your preferred convention, no?2012-02-15
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    I just happen to be a TA in a calculus course this semester and we just got to spherical coordinates. We teach that $0\leq\phi\leq\pi$ and $z=r\cos\phi$.2012-06-17

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