Vanishing line — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
The horizon, and the fraction
The horizon crosses every upright at the point of it that stands at the camera's own eye height — always, whatever the picture plane is doing. It crosses at the same *fraction* of the drawn height only when the plane is vertical: tilt by 6° and the fractions spread by 0.08 percentage points, tilt by 4° and 0.06. One statement is an incidence and survives; the other is a ratio and does not.
The ramp has its own horizon
Every plane has a vanishing line, and a ramp's is not the ground's. Its uphill edges meet on a line above the horizon, and the angle at the eye between that meeting point and the same direction taken level is the gradient — 22.0000° recovered against 22° built, out of the picture alone, with no scale, no ruler and nothing known about the scene except that the ground is level.
Named alongside it
The objects these essays reach for when they reach for this one.
HorizonPicture planepoint at infinitysingle-view metrologyAngle from vanishing pointsCross ratioEye levelFocal lengthForeshorteningGradientInclined planenecessary, not sufficient