40 Computing in Geographic Information Systems
These choices determine the reference plane that contains the origin and is
perpendicular to the zenith. The spherical coordinates of a point P are then
defined as follows:
1. The radius or radial distance ‘r’ which is the Euclidean distance
from the origin O to P.
2. The inclination θ (or polar angle) is the angle between the zenith
direction and the line segment OP.
3. The azimuth φ (or azimuthal angle) is the signed angle measured
from the azimuth reference direction to the orthogonal projection
of the line segment OP on the reference plane.
4. 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