radial [PHOTOGRAMMETRY)—A line or direction from the radial center to any point on a photograph. The radial center is assumed to be the principal point unless otherwise designated as, for example, nadir radial, i.e., a radial from the nadir point, or iso-radial, which is a radial from the isocenter.

radial, radial line—A radius line of a circular cunt to a designated point on the curve. If the line is extended beyond the convex side of the curve, it is a prolongation of the radial line.

radial center—The selected point on a photograph from which radials (directions) to various image points are drawn or measured; origin of radials. The radial center is either the principal point, the nadir point, the isocenter, or a substitute center.

radian—An angle of 360°/2π (or approximately 57° 17′ 44.8″) subtended at the center of a circle by an arc equal in length to the radius of the circle.

–

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

Part of LearnCST’s exam text bundle.