Spherical coordinates

[SammyS]

Is the Wikipedia article posted earlier by @Mark44 wrong about $\rho$ being negative in some cases?

It's not always positive, it could be zero.

 

[pbuk]

The point is you can use any system for spherical coordinates as long as it is well defined. For instance every time you get in an aircraft you rely on a system which uses ±90° latitude, ±180° longitude and height above (or below, in which case you are in trouble) an approximately equipotential geoid.