is perpendicular to the zenith. The spherical coordinates of a point are then defined as follows: The radius or radial distance is the Euclidean distance from the origin to . The azimuth (or azimuthal angle) is the signed angle measured from the azimuth reference direction to the orthogonal projection of the line segment on the reference plane. The inclination (or polar angle) is the angle between the zenith direction and the line segment . The sign of the azimuth is determined by choosing what is a positive sense of turning about the zenith. This choice is arbitrary, and is part of the coordinate system's definition. The elevation angle is the signed angle between the reference plane and the line segment , where positive angles are oriented towards the zenith. Equivalently, it is 90 degrees ( radians) minus the inclination angle. If the inclination is zero or 180 degrees ( radians), the azimuth is arbitrary. If the radius is zero, both azimuth and inclination are arbitrary. In linear algebra, the vector from the origin to the point is often called the position vector of P. Conventions Several different conventions exist for representing the three coordinates, and for the order in which they should be written. The use of to denote radial distance, inclination (or elevation), and azimuth, respectively, is common practice in physics, and is specified by ISO standard 80000-2:2019, and earlier in ISO 31-11 (1992). However, some authors (including mathematicians) use ρ for radial distance, φ for inclination (or elevation) and θ for azimuth, and r for radius from the z-axis, which "provides a logical extension of the usual polar coordinates notation". Some authors may also list the azimuth before the inclination (or elevation). Some combinations of these choices result in a left-handed coordinate system. The standard convention conflicts with the usual notation for two-dimensional polar coordinates and three-dimensional cylindrical coordinates, where is often used for the azimuth. The angles are typically measured in degrees (°) or radians (rad), where 360° = 2 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. 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