(1) The angle, at the center of a circular arc, included between the radii which pass through the beginning and end of the arc. This angle is equal in size to the change in direction of the tangents at the beginning and end points. In alinement surveys, it is commonly called the delta angle and denoted by D. (2) The angle, in curved lines containing segments of compound curves, or spirals and circular curves, between the radii to the beginning and end points or between the tangents at these points. The central angle of a spiral is called theta (q) or delta (D).