What is the primary surface in airfield operations centered on?

The primary surface in airfield operations is centered on the runway because the runway is the primary area used for aircraft takeoffs and landings. It is designed specifically to support the weight and speed of aircraft during these critical phases of flight. The runway's dimensions, surface materials, and markings are all established to ensure the safety and efficiency of air operations, making it the focal point of airfield activity.

In contrast, while the taxiway is essential for guiding aircraft to and from the runway, it does not serve the same function as the runway itself. The parking area, or ramp, provides space for aircraft when they are not in flight, but it is not involved in takeoff or landing operations. The terminal area refers to the vicinity around the airport where operations such as passenger boarding, baggage handling, and other services occur, but again, it does not play the primary role of facilitating the aircraft's takeoff and landing. Therefore, the focus on the runway highlights its critical role in airfield operations.