Instrument Flying Handbook Menu>Human Factors>Optical Illusions>Runway and Terrain Slopes Illusion

An upsloping runway, upsloping terrain, or both, can create an illusion the aircraft is at a higher altitude than it actually is. [Figure 1-5B] The pilot who does not recognize this illusion will fly a lower approach. Downsloping runways and downsloping approach terrain can have the opposite effect.

Figure 1-5. Runway width and slope illusions.