The point of an aerial photograph that lies at the optical axis of the camera, defined by the intersection of the lines drawn from the fiducial marks. The point to which the camera was pointing at the time of exposure.
One of the two points that each lens has along the optical axis. The principal point closer to the imaging device (CCD chip in our case) is used as a reference point when measuring the focal length of a lens.
The focal length of a thin, double-convex, single-element lens is the distance along the optical axis from the center of the lens to its focal point. This center point of the lens is called the principal point. However, since actual photographic lenses consist of combinations of several convex and concave lens elements, it is not visually apparent where the center of the lens might be. The principal point of a multi-element lens is therefore defined as the point on the optical axis at a distance equal to the focal length measured back toward the lens from the focal point. The principal point measured from the front focal point is called the front principal point, and the principal point measured from the rear focal point is called the rear principal point. The distance between these two principal points is called the principal point interval.
(1) The intersection of two lines drawn through pairs of opposite fiducial marks on an aerial photograph. (2) The theoretical intersection of the camera's line of sight axis, directed as vertically as possible, with the ground.
A point from where the focal length of the lens is measured. Normally located at the center of the lens. However, compound lenses have two principal points, and the location of these principal points cannot be determined by appearance.
The point on the earth where a satellite sensor is focused at any time during its orbit. If the sensor vertical axis is perpendicular to the earth's surface, the principal point coincides with the subpoint. 2. A term used in remote sensing; the point where the optical axis intersects the principal plane.
point from which the focal length is measured. The principal point of a simple lens is located at the center of the lens. Compound lenses have two principal points, the location of which cannot be determined by appearance.