What is indicated by the counter on an altimeter display?

The counter on an altimeter display specifically indicates altitude in feet above sea level. An altimeter measures the atmospheric pressure and correlates it to altitude through a standard barometric pressure reference. As altitude increases, the atmospheric pressure decreases; the altimeter uses this change in pressure to determine the altitude and displays it in feet. This measurement is crucial for pilots and aircrew to ensure safe navigation and appropriate flight levels.

While pressure altitude is a related concept, it is specifically tied to a standardized atmospheric pressure setting, typically 29.92 inches of mercury. The altimeter, however, shows the actual altitude based on current atmospheric conditions. Understanding how an altimeter functions is fundamental for safe flight operations, especially when terrain varies significantly.