<P> Several different conventions exist for representing the three coordinates, and for the order in which they should be written . The use of (r, θ, φ) to denote radial distance, inclination (or elevation), and azimuth, respectively, is common practice in physics, and is specified by ISO standard 80000 - 2: 2009, and earlier in ISO 31 - 11 (1992). </P> <P> However, some authors (including mathematicians) use φ for inclination (or elevation) and θ for azimuth, which "provides a logical extension of the usual polar coordinates notation". Some authors may also list the azimuth before the inclination (or elevation), and / or use ρ (rho) instead of r for radial distance . Some combinations of these choices result in a left - handed coordinate system . The standard convention (r, θ, φ) conflicts with the usual notation for the two - dimensional polar coordinates, where θ is often used for the azimuth . It may also conflict with the notation used for three - dimensional cylindrical coordinates . </P> <P> The angles are typically measured in degrees (°) or radians (rad), where 360 ° = 2π rad . Degrees are most common in geography, astronomy, and engineering, whereas radians are commonly used in mathematics and theoretical physics . The unit for radial distance is usually determined by the context . </P> <P> When the system is used for physical three - space, it is customary to use positive sign for azimuth angles that are measured in the counter-clockwise sense from the reference direction on the reference plane, as seen from the zenith side of the plane . This convention is used, in particular, for geographical coordinates, where the "zenith" direction is north and positive azimuth (longitude) angles are measured eastwards from some prime meridian . </P>

Spherical coordinate θ is related to the cylindrical coordinate as