Vanishing point of multipoint perspective, and historian Kirsti Andersen compiled these observations. She notes, in terms of projective geometry, the vanishing point is the image of the point at infinity associated with , as the sightline from through the vanishing point is parallel to . As a vanishing point originates in a line, so a vanishing line originates in a plane that is not parallel to the picture . Given the eye point , and the plane parallel to and lying on , then the vanishing line of is . For example, when is the ground plane and is the horizon plane,
Point-to-point transit or connecting flights so baggage must be collected and rechecked even to transfer between flights booked at the same time on the same airline. Although there are many point-to-point airlines, most have at least a "homebase" airport where most flights originate or depart. The United States airport system was point-to-point, controlled by CAB, until deregulation late 1960s/early 1970s, and eventually the 1978 Airline Deregulation Act when they switched to the hub concept, until the advent of low-cost carriers, when point-to-point air transport increased. Some full-service network carriers operate the point-to-point model alongside the hub-and-spoke system for certain high-density routes between