The correct answer is D. All the above.
On vertical photographs, height displacement is positive for points above datum, negative for points below datum, and zero for points vertically below the air station.
Datum is a reference surface used in surveying and mapping. It is usually a horizontal plane, such as mean sea level, that is used to measure the elevations of points on the Earth’s surface.
Height displacement is the difference between the elevation of a point on the Earth’s surface and its elevation on a vertical photograph. The elevation of a point on a vertical photograph is the distance from the point to the optical center of the camera lens.
For points above datum, the height displacement is positive. This is because the points are located above the datum plane, and the distance from the points to the optical center of the camera lens is greater than the elevation of the datum plane.
For points below datum, the height displacement is negative. This is because the points are located below the datum plane, and the distance from the points to the optical center of the camera lens is less than the elevation of the datum plane.
For points vertically below the air station, the height displacement is zero. This is because the points are located on the datum plane, and the distance from the points to the optical center of the camera lens is equal to the elevation of the datum plane.