Define flying height

Flying height is the vertical distance of the aircraft (the camera lens at the moment of exposure) above a given datum, usually mean sea level, or above the average ground level. For flat ground at elevation $h$ above datum, the photo scale is $S = f/(H - h)$ (or simply $S = f/H$ when $H$ is measured above the ground), so for a given camera a greater flying height gives a smaller scale.